[Question]
  [
Write a program or function that listens for incoming TCP traffic on port N. It offers a simple service: it calculates sum of IP address fields of incoming connection and returns.
Program or function reads integer N from arguments or stdin. It listens to incoming TCP connections on port N. When someone connects to that port, the program calculates sum of its IP address fields and sends it back to the client with trailing newline and closes connection.
* Port number N is a valid port, and 210 < N < 215
* Trailing newline can be either `\n`or `\r\n`
* You can use either IPv4 or IPv6. Since IPv6 addresses are written in hexadecimal form, you must also provide result in same format, for example `2001:0db8:0000:0042:0000:8a2e:0370:7334 => 12ecd`.
This is [code-golf](/questions/tagged/code-golf "show questions tagged 'code-golf'"). Standard rules and loopholes apply.
## Example
You run your server with `./server 1234`. The server is now running and waiting for connections on port 1234. Then a client from `127.0.0.1` connects to your server. Your server performs a simple calculation: `127+0+0+1 => 128` and sends the result to the client (with trailing newline): `128\n`. Then server closes connection and waits for next client.
## Leaderboard
```
var QUESTION_ID=76379,OVERRIDE_USER=20569;function answersUrl(e){return"https://api.stackexchange.com/2.2/questions/"+QUESTION_ID+"/answers?page="+e+"&pagesize=100&order=desc&sort=creation&site=codegolf&filter="+ANSWER_FILTER}function commentUrl(e,s){return"https://api.stackexchange.com/2.2/answers/"+s.join(";")+"/comments?page="+e+"&pagesize=100&order=desc&sort=creation&site=codegolf&filter="+COMMENT_FILTER}function getAnswers(){jQuery.ajax({url:answersUrl(answer_page++),method:"get",dataType:"jsonp",crossDomain:!0,success:function(e){answers.push.apply(answers,e.items),answers_hash=[],answer_ids=[],e.items.forEach(function(e){e.comments=[];var s=+e.share_link.match(/\d+/);answer_ids.push(s),answers_hash[s]=e}),e.has_more||(more_answers=!1),comment_page=1,getComments()}})}function getComments(){jQuery.ajax({url:commentUrl(comment_page++,answer_ids),method:"get",dataType:"jsonp",crossDomain:!0,success:function(e){e.items.forEach(function(e){e.owner.user_id===OVERRIDE_USER&&answers_hash[e.post_id].comments.push(e)}),e.has_more?getComments():more_answers?getAnswers():process()}})}function getAuthorName(e){return e.owner.display_name}function process(){var e=[];answers.forEach(function(s){var r=s.body;s.comments.forEach(function(e){OVERRIDE_REG.test(e.body)&&(r="<h1>"+e.body.replace(OVERRIDE_REG,"")+"</h1>")});var a=r.match(SCORE_REG);a&&e.push({user:getAuthorName(s),size:+a[2],language:a[1],link:s.share_link})}),e.sort(function(e,s){var r=e.size,a=s.size;return r-a});var s={},r=1,a=null,n=1;e.forEach(function(e){e.size!=a&&(n=r),a=e.size,++r;var t=jQuery("#answer-template").html();t=t.replace("{{PLACE}}",n+".").replace("{{NAME}}",e.user).replace("{{LANGUAGE}}",e.language).replace("{{SIZE}}",e.size).replace("{{LINK}}",e.link),t=jQuery(t),jQuery("#answers").append(t);var o=e.language;/<a/.test(o)&&(o=jQuery(o).text()),s[o]=s[o]||{lang:e.language,user:e.user,size:e.size,link:e.link}});var t=[];for(var o in s)s.hasOwnProperty(o)&&t.push(s[o]);t.sort(function(e,s){return e.lang>s.lang?1:e.lang<s.lang?-1:0});for(var c=0;c<t.length;++c){var i=jQuery("#language-template").html(),o=t[c];i=i.replace("{{LANGUAGE}}",o.lang).replace("{{NAME}}",o.user).replace("{{SIZE}}",o.size).replace("{{LINK}}",o.link),i=jQuery(i),jQuery("#languages").append(i)}}var ANSWER_FILTER="!t)IWYnsLAZle2tQ3KqrVveCRJfxcRLe",COMMENT_FILTER="!)Q2B_A2kjfAiU78X(md6BoYk",answers=[],answers_hash,answer_ids,answer_page=1,more_answers=!0,comment_page;getAnswers();var SCORE_REG=/<h\d>\s*([^\n,]*[^\s,]),.*?(\d+)(?=[^\n\d<>]*(?:<(?:s>[^\n<>]*<\/s>|[^\n<>]+>)[^\n\d<>]*)*<\/h\d>)/,OVERRIDE_REG=/^Override\s*header:\s*/i;
```
```
body{text-align:left!important}#answer-list,#language-list{padding:10px;width:290px;float:left}table thead{font-weight:700}table td{padding:5px}
```
```
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <link rel="stylesheet" type="text/css" href="//cdn.sstatic.net/codegolf/all.css?v=83c949450c8b"> <div id="answer-list"> <h2>Leaderboard</h2> <table class="answer-list"> <thead> <tr><td></td><td>Author</td><td>Language</td><td>Size</td></tr></thead> <tbody id="answers"> </tbody> </table> </div><div id="language-list"> <h2>Winners by Language</h2> <table class="language-list"> <thead> <tr><td>Language</td><td>User</td><td>Score</td></tr></thead> <tbody id="languages"> </tbody> </table> </div><table style="display: none"> <tbody id="answer-template"> <tr><td>{{PLACE}}</td><td>{{NAME}}</td><td>{{LANGUAGE}}</td><td>{{SIZE}}</td><td><a href="{{LINK}}">Link</a></td></tr></tbody> </table> <table style="display: none"> <tbody id="language-template"> <tr><td>{{LANGUAGE}}</td><td>{{NAME}}</td><td>{{SIZE}}</td><td><a href="{{LINK}}">Link</a></td></tr></tbody> </table>
```
  
[Answer]
# Bash + netcat + ss + …, ~~65~~ 60 characters
```
nc -lp$1 -c'ss src :'$1'|awk \$0=\$5|tr .: +#|bc'
exec $0 $1
```
Not a serious solution, was just curious about this possibility.
Thanks to:
* [ninjalj](https://codegolf.stackexchange.com/users/267/ninjalj) for suggesting the `awk` based filtering (-5 characters)
Sample run:
(terminal 1)
```
bash-4.3$ ./ip-reduce.sh 8080
```
(terminal 2)
```
bash-4.3$ nc localhost 8080
128
bash-4.3$ telnet localhost 8080
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
128
Connection closed by foreign host.
```
On Ubuntu you can get `nc` from [netcat-traditional](http://packages.ubuntu.com/wily/netcat-traditional) (no, netcat-openbsd is not good) and `ss` from [iproute2](http://packages.ubuntu.com/wily/iproute2).
[Answer]
## Linux ELF/x86, 146 bytes
```
00000000  7f 45 4c 46 01 00 00 00  5a 0e 00 00 5a 5e eb 10  |.ELF....Z...Z^..|
00000010  02 00 03 00 0c 50 eb 10  0c 50 eb 10 04 00 00 00  |.....P...P......|
00000020  5e 53 43 53 52 89 e1 b0  66 3d 20 00 01 00 cd 80  |^SCSR...f= .....|
00000030  97 55 6b ed 0a 01 c5 ac  2c 30 79 f6 43 0f cd 01  |.Uk.....,0y.C...|
00000040  dd 55 89 e1 6a 10 51 6a  10 51 57 89 e1 b0 66 cd  |.U..j.Qj.QW...f.|
00000050  80 43 43 b0 66 cd 80 01  64 24 08 89 e1 43 b0 66  |.CC.f...d$...C.f|
00000060  cd 80 89 c1 93 8d 74 24  1b 99 fd ac 01 c2 e2 fb  |......t$........|
00000070  89 f7 b0 0a aa 91 92 f6  f1 86 c4 04 30 aa 42 c1  |............0.B.|
00000080  e8 08 75 f3 42 89 f9 41  b0 04 cd 80 b0 06 cd 80  |..u.B..A........|
00000090  eb c9                                             |..|
00000092
```
Includes a 52-byte ELF header, a 32-byte program header, 111 bytes of program code + 3 bytes of code for skipping around inside the headers.
Information on how to create tiny ELF executables can be found at [breadbox](https://codegolf.stackexchange.com/users/4201/breadbox)'s *"[A Whirlwind Tutorial on Creating Really Teensy ELF Executables for Linux](http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html)"*.
Linux/i386 uses the [`socketcall(2)`](http://man7.org/linux/man-pages/man2/socketcall.2.html) multiplex system call, which takes in `ebx` the specific socket call (the `SYS_*` macros from `/usr/include/linux/net.h`), and in `ecx` a pointer to the argument area of the original library call.
Some things done to keep the executable small:
* It assumes registers are zeroed on entry, which Linux does, but is not required by the ELF standard (the only requirement is that on entry `EDX` points to a finalization funtion (useful for executables loaded by the dynamic linker) or is NULL).
* It assumes on launch (typically by a shell) the only open file descriptors are 0, 1, and 2. Which means that the listening socket will be fd 3, and the accepted socket will be fd 4.
* It assumes there are exactly 2 arguments (including `argv[0]`).
* The same stack space is reused for the calls to `bind(2)`, `listen(2)` and `accept(2)`.
* To skip over the `phentsize` and `phnum` fields, a byte is prepended, turning into a `CMP` operation which takes the `phentsize` and `phnum` fields as an immediate (trick shamelessly stolen from [breadbox's solution to 123 in anarchy golf](http://golf.shinh.org/reveal.rb?123/breadbox_1180162215&out)).
* x86 string operations `LODS` (load into accumulator and increment/decrement source index) and `STOS` (store from accumulator and increment/decrement destination index) are nice for short code.
* `XCHG EAX, reg` is 1-byte, compared to `MOV EAX, reg`, which takes 2 bytes.
* `CDQ/CLTD` (sign-extend `EAX` into `EDX:EAX`) can be used as a 1-byte way to zero the `EDX` register.
* `BSWAP` is useful for implementing `htons()`.
Nasm source:
```
BITS 32                                         ;
                                                ;   ELF HEADER    --   PROGRAM HEADER
; ELF HEADER                                    ; +-------------+
DB 0x7f,'E','L','F'                             ; | magic       |    +--------------------+
                                                ; |             |    |                    |
; PROGRAM HEADERS                               ; |             |    |                    |
DD 1                                            ; |*class   32b | -- | type: PT_LOAD      |
                                                ; |*data   none |    |                    |
                                                ; |*version   0 |    |                    |
                                                ; |*ABI    SysV |    |                    |
DD 0xe5a        ; offset = vaddr & (PAGE_SIZE-1); |*ABI vers    | -- | offset             |
                                                ; |             |    |                    |
entry:  pop     edx     ; edx = 2 (argc)        ; |*PADx7       | -- | vaddr = 0x10eb5e5a |
        pop     esi     ; discard argv[0]       ; |             |    |                    |
        jmp     short skip                      ; |             |    |                    |
DW 2                                            ; | ET_EXEC     | -- |*paddr LO           |
DW 3                                            ; | EM_386      | -- |*paddr HI           |
DD 0x10eb500c                                   ; |*version     | -- | filesz             |
DD 0x10eb500c                                   ; | entry point | -- | memsz              |
DD 4                                            ; | ph offset   | -- | flags: RX          |
                                                ; |             |    |                    |
skip:   pop     esi     ; esi = argv[1]         ; |*sh offset   | -- |*align              |
socket: push    ebx     ; default protocol (0)  ; |             |    |                    |
        inc     ebx                             ; |             |    |                    |
        push    ebx     ; SOCK_STREAM (1)       ; |             |    |                    |
        push    edx     ; AF_INET (2)           ; |*flags       |    +--------------------+
        mov     ecx, esp                        ; |             |
        mov     al, 0x66                        ; |*ehsize      |
DB 0x3d         ; cmp eax,0x10020               ; |             |
DW 32                                           ; | phentsize   |
DW 1                                            ; | phnum       |
                                                ; |             |
        int     0x80    ; socket(2, 1, 0)       ; |*shentsize   |
        xchg    edi, eax; edi = sockfd, eax = 0 ; |*shnum       |
        push    ebp     ; INADDR_ANY            ; |             |
                                                ; |             |
mult:   imul    ebp, 10 ; \_                    ; |*shstrndx    |
        add     ebp, eax; >                     ; |             |
        lodsb           ; >                     ; +-------------+
        sub     al,'0'  ; >
        jns     mult    ; / ebp = atoi(argv[1])                 ;       bind stack frame
                                                                ;    +-----------------------+
endmul: inc     ebx             ; SYS_BIND (2)                  ;    |        INADDR_ANY     |
                                                                ; +->| AF_INET | htons(port) |
        bswap   ebp                                             ; |  +-----------------------+
        add     ebp, ebx        ; AF_INET (2), htons(port)      ; |  |           16          |
        push    ebp                                             ; |  +-----------------------+
                                                                ; |  |         dummy         |
        mov     ecx, esp                                        ; |  +-----------------------+
        push    16              ; addrlen                       ; |  |           16          |
        push    ecx             ; dummy value                   ; |  +-----------------------+
        push    16              ; addrlen                       ; +--|          addr         |
        push    ecx             ; addr                          ;    +-----------------------+
        push    edi             ; sock                          ;    |         sockfd        |
        mov     ecx, esp                                        ;    +-----------------------+
        mov     al, 0x66
        int     0x80            ; bind(sockfd, addr, addrlen)
                                                                ;       accept stack frame
                                                                ;    +-----------------------+
listen: ;mov    byte [esp+8],1                                  ;    |        INADDR_ANY     |
        inc     ebx                                             ; +->| AF_INET | htons(port) |
        inc     ebx             ; SYS_LISTEN (4)                ; |  +-----------------------+
        mov     al, 0x66                                        ; |+>|           16          |
        int     0x80            ; listen(sockfd, backlog)       ; || +-----------------------+
                                                                ; || |         dummy         |
        add     [esp+8], esp                                    ; || +-----------------------+
accept: mov     ecx, esp                                        ; |+-|        &addrlen       |
        inc     ebx             ; SYS_ACCEPT (5)                ; |  +-----------------------+
        mov     al, 0x66                                        ; +--|          addr         |
        int     0x80            ; accept(sockfd, addr, &addrlen);    +-----------------------+
                                                                ;    |         sockfd        |
        mov     ecx, eax        ; ecx = 4                       ;    +-----------------------+
        xchg    ebx, eax        ; ebx = acceptfd, eax = 000000xx
        lea     esi, [esp+27]   ; point to the IP part of struct sockaddr_in
        cdq
        std                     ; reverse direction for string operations
addip:  lodsb                   ; \_
        add     edx, eax        ; > edx = sum of 4 IP bytes
        loop    addip           ; /
        mov     edi, esi        ; reuse struct sockaddr_in as scratch buffer
        mov     al, 10          ; '\n'
        stosb
        xchg    ecx, eax        ; ecx = 10
        xchg    eax, edx        ; edx = 0, eax = sum
divide: div     cl              ; \_
        xchg    al, ah          ; >
        add     al,0x30         ; >
        stosb                   ; > sprintf(scratch, "%d", sum)
        inc     edx             ; >
        shr     eax, 8          ; >
        jnz     divide          ; /
write:  inc     edx             ; ndigits + 1 ('\n')
        mov     ecx, edi
        inc     ecx
        mov     al,4
        int     0x80            ; write(acceptfd, scratch, scratchlen) 
close:  mov     al, 6
        int     0x80            ; close(acceptfd)
        jmp     accept
```
[Answer]
# C#, ~~284~~ ~~283~~ ~~282~~ ~~278~~ ~~274~~ 254 bytes
```
class A{static int Main(string[]a){var b=new System.Net.Sockets.TcpListener(int.Parse(a[0]));b.Start();for(;;){var c=b.AcceptTcpClient();var d=c.Client.LocalEndPoint.Serialize();new System.IO.StreamWriter(c.GetStream()).WriteLine(d[4]+d[5]+d[6]+d[7]);}}}
```
Classic example of a basic C# TCP server. Testing:
**Terminal 1:**
```
$ ./Q76379.exe 1029
```
**Terminal 2:**
```
$ telnet localhost 1029
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
128
Connection closed by foreign host.
```
**Firefox:**
![](https://i.stack.imgur.com/Xv2je.png)
[Answer]
# NodeJS, ~~146~~ ~~134~~ 127 bytes
```
require('http').createServer((q,s)=>s.end(eval(0+q.socket.remoteAddress.replace(/^.*:|\./g,'+'))+'\n')).listen(process.argv[2])
```
I finally get to post a NodeJS answer! IPv4 only right now.
Sample execution: `node script.js 1024`. From another terminal:
```
$ curl 127.0.0.1:1024
128
```
[Answer]
# Tcl, 92
* 1 byte saved thanks to @DonalFellows.
```
proc s {c a p} {puts $c [expr [string map .\ + $a]]
close $c}
socket -server s $argv
vwait f
```
Fairly self-explanatory:
`socket -server s $argv` creates a listening socket on the port specified in the arguments.
Every time a new connection arrives, the `proc s` is called, with channel, source-address and source-port as parameters. `string map` substitutes `.` for `+` in the source address, and `expr` arithmetically evaluates the result, which is then `puts` back to the connection channel `c`.
`vwait` runs an event loop to catch the incoming connection events.
---
Credit to @DonalFellows for the following:
Here's a version that handles IPv6 (requires Tcl 8.6; most of the extra length is due to producing a hex response):
# Tcl, 109
```
proc s {c a p} {puts $c [format %x [expr 0x[string map :\ +0x0 $a]]]
close $c}
socket -server s $argv
vwait f
```
[Answer]
# Groovy 133, 125, 93, 89
```
new ServerSocket(args[0]as int).accept{it<<(it.inetAddress.address as int[]).sum()+"\n"}
```
IPv4 only, probably.
Ungolfed:
```
new ServerSocket(args[0]as int).accept{
    it << (it.inetAddress.address as int[]).sum()+"\n"
}
```
Testing:
```
$ telnet localhost 9000
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
128
Connection closed by foreign host.
```
[Answer]
# Python 3, 170 166 147 bytes
```
from socket import*
s=socket()
s.bind(("",int(input())))
s.listen()
while 1:
 c,a=s.accept()
 c.send(b"%d\n"%eval(a[0].replace(".","+"))),c.close()
```
Takes port on `stdin`, IPv4 only. Works on GNU/Linux (and, I assume, most other unices), which automatically expands "" to "0.0.0.0", not sure about Windows though.
[Answer]
# Java, 371 368 350 344 333 310 295 282 bytes
## Golfed
```
import java.net.*;class A{public static void main(String[]n)throws Exception{ServerSocket s=new ServerSocket(Integer.decode(n[0]));for(;;){try(Socket a=s.accept()){byte[]c=a.getInetAddress().getAddress();new java.io.PrintStream(a.getOutputStream()).println(c[0]+c[1]+c[2]+c[3]);}}}}
```
### Ungolfed
```
import java.net.*;
class A {
    public static void main(String[] n) throws Exception {
        ServerSocket s = new ServerSocket(Integer.decode(n[0]));
        for (;;) {
            try (Socket a = s.accept()) {
                byte[] c = a.getInetAddress().getAddress();
                new java.io.PrintStream(a.getOutputStream()).println(c[0] + c[1] + c[2] + c[3]);
            }
        }
    }
}
```
### Output
```
mallard@steamroller:~$ telnet localhost 8888
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
128
Connection closed by foreign host.
```
[Answer]
## PowerShell v2+, ~~303~~ ~~268~~ ~~257~~ 227 bytes
```
nal n new-object;($l=n Net.Sockets.TcpListener($args[0])).Start()
for(){($w=n IO.StreamWriter(($c=$l.AcceptTcpClient()).GetStream())).Write((([Net.IPEndPoint]$c.Client.RemoteEndPoint).Address-replace"\.",'+'|iex))
$w.Dispose()}
```
Saved 35 bytes thanks to [Matt](https://codegolf.stackexchange.com/users/52023/matt) ... Saved another 11 bytes by aliasing `New-Object` and minor tweaks ... Saved 30 more bytes by implicitly using localhost rather than `any` IP address and corrected to account for continual usage as originally specified and I missed
Really similar to the [C# answer](https://codegolf.stackexchange.com/a/76380/42963), since it's .NET underlying both. We save a few bytes here over the C# answer by being able to leverage the returning functionality of PowerShell (surrounding our declaration/assignment in parens, and then immediately calling the . methods), but we lose an awful lot by needing to formulate the summation. The fact that we've got slightly shorter class names/calls is really why this answer is beating C#.
### Explanation
We first create a `New-Alias` (with the `nal` alias) to save on re-typing `New-Object` later. The rest of the first line is setting up a TCP Listener. We pass the command-line `$args[0]` as the input for creating a new `System.Net.Sockets.TcpListener`, stored as `$l`. That object is encapsulated in parens and immediately called with `.Start()` to have it actively open the socket.
Entering an infinite `for` loop, we then set our listener `$l` to blocking with `AcceptTcpClient()` which will wait for a connection. A reference to that connection (once it's made) is stored in `$c`, encapsulated in parens, and immediately called `GetStream()` to get the datastream. That datastream is passed to a new `System.IO.StreamWriter` constructor, `$w`, so we can manipulate it. That constructor is itself encapsulated in parens and immediately called `Write(...)`.
Inside the `Write(...)` call, we take our client handle `$c` and obtain the client's `RemoteEndPoint` property. This is the only way (that I've found so far) to get the remote IP address. Next we need to re-cast that as a `[System.Net.IPEndPoint]` object so it's formatted correctly, encapsulate that in parens, and pull just the `.Address` property. We then `-replace` the literal periods with plus signs, then pipe it to `Invoke-Expression` (similar to `eval`) to obtain our summation.
After the IO write, we need to call `.Dispose()` to ensure the datastream is flushed to the client and closed. The TCP server drops the client connection without warning, so depending upon the client used it may hang for a while at this point. It then continues through the `for` loop without properly closing connections. This also means that it leaks memory and system handles like crazy, but we don't care about that, right? You may need to use Task Manager to kill the process when you're done running the server, though. :D
Also IPv4 only, as the summation barfs spectacularly trying to handle an IPv6 address, since `:` isn't a valid algebraic operator for `iex` to parse.
[Answer]
# PHP, 161 (56?)
This is my first post in here. I hope this goes right :)
```
<?php $s=socket_create_listen($argv[1]);while($c=socket_accept($s)){socket_getpeername($c,$r);socket_write($c,array_sum(explode('.',$r))."\n");socket_close($c);}
```
**Ungolfed:**
```
<?php 
    $s = socket_create_listen($argv[1]); //Create socket
    while( $c = socket_accept($s) ) { // Loop accepting new connections
        socket_getpeername($c, $r); // Get IP address in $r
        socket_write($c, array_sum(explode('.', $r))."\n"); //Calculate sum
        socket_close($c); //Close connection and wait for next one
    }
```
**Terminal:**
```
$ php test.php 8080 &
$ telnet localhost 8080
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
128
Connection closed by foreign host.
```
This only works for IPV4
**Edit**:
I just noticed that php supports basic server:  
I decided stick to original character count unless somebody confirms if following is allowed :)
test2.php: (possible 56-byte solution)
```
<?=array_sum(explode('.',$_SERVER['REMOTE_ADDR']))."\n";
```
And then start service with:
```
php -S localhost:8080 test2.php
```
**Chrome as client**
[![screenshot](https://i.stack.imgur.com/VAaUT.png)](https://i.stack.imgur.com/VAaUT.png)
**Edit 2: wget as client**
```
$ wget -qO- localhost:8080
128
```
[Answer]
# [Go](https://golang.org/), ~~359~~ 311
This is my first program in Go - It allowed to let me discover one thing : This is definitively not a good golfing language!
(Kudos to @steve who did most of the golfing !)
```
package main
import(n"net";t"strings";r"strconv";x"regexp";"os")
func main(){l,_:=n.Listen("tcp",":"+os.Args[1])
for{c,_:=l.Accept();var s int
for _,i:=range t.Split(x.MustCompile(":[0-9]+$").ReplaceAllLiteralString(c.RemoteAddr().String(),""),"."){
n,_:=r.Atoi(i);s=s+n};c.Write([]byte(r.Itoa(s)));c.Close()}}
```
[Answer]
## Common Lisp, 110 bytes
```
(use-package'usocket)(lambda(p)(socket-server"localhost"p(lambda(u)(format u"~D~%"(reduce'+ *remote-host*)))))
```
## Details
```
(use-package 'usocket)
(lambda (port)
  ;; create server with event-loop
  (socket-server "localhost"
                 port
                 ;; tcp-handler
                 (lambda (stream)
                   ;; format to stream to client
                   (format stream
                           "~D~%"
                           ;; add all elements of the host,
                           ;; a vector of 4 integers
                           (reduce #'+ *remote-host*))
                   ;; client connection is closed automatically
                   ;; when exiting this function                     
                 )))
```
[Answer]
# q, 88 bytes
```
system raze"p ",1_.z.x;.z.pg:{(string sum"i"$0x0 vs .z.a),"\n"};.z.ph:{.h.hy[`;.z.pg[]]}
```
* `system raze"p ",1_.z.x`: Takes the second command-line argument (the first `"-"` is for telling `q` not to interpret `N` as a script/file) and opens a port (`"p "`) with it.
	+ Note: Calling `q -p N` sets the port as `N` automatically, but since the question seems to suggest that `N` should be an argument to the program rather than the executable itself, I've gone the longer way.
* Inside the `.z.pg` function that handles incoming requests, `.z.a` holds the IP address as a 32-bit integer.
	+ `"i"$0x0 vs` splits it into its integer 'constituents', and `sum` does the summation.
	+ Last, `string` the numeric result and append `"\n"` to it to return to the client.
* `.z.ph` is another function for HTTP GET requests, with extra handling to convert the string output into a valid HTTP response.
Demo - Server:
```
c:\q\w32>q - 1234
KDB+ 3.3 2015.11.03 Copyright (C) 1993-2015 Kx Systems
w32/ 4()core ... NONEXPIRE
Welcome to kdb+ 32bit edition
q)system raze"p ",1_.z.x;.z.pg:{(string sum"i"$0x0 vs .z.a),"\n"};.z.ph:{.h.hy[`;.z.pg[]]}
q)
```
Demo - Client (from another `q` session running on `127.0.0.1`):
```
q)(hopen `::1234)""
"128\n"
```
Demo - Client (from `curl`):
```
$ curl localhost:1234
128
$
```
[Answer]
# 8086 machine code (16-bit DOS), ~~163 156 148 148~~ 142 bytes
```
00000000  31 c0 bb 0a 00 31 c9 be  81 00 bf 80 00 8a 0d 01  |1....1..........|
00000010  cf 46 8a 0c 80 e9 30 f7  e3 00 c8 39 fe 72 f2 89  |.F....0....9.r..|
00000020  c3 89 c1 b8 01 10 ba ff  ff 31 f6 31 ff cd 61 53  |.........1.1..aS|
00000030  b8 00 12 bf 80 00 b9 01  00 ba ff ff cd 61 b8 00  |.............a..|
00000040  14 cd 61 31 c0 bb 0a 00  83 c7 06 8d 4d 04 26 02  |..a1........M.&.|
00000050  05 80 d4 00 47 39 cf 72  f5 bf 84 00 b9 80 00 bb  |....G9.r........|
00000060  0a 00 4f 31 d2 f7 f3 80  c2 30 88 15 39 cf 77 f2  |..O1.....0..9.w.|
00000070  1e 07 b8 0e 13 5b bf 80  00 b9 04 00 ba ff ff cd  |.....[..........|
00000080  61 b8 00 11 ba 01 00 cd  61 b8 00 4c cd 21        |a.......a..L.!|
0000008e
```
Equivalent assembly code:
```
org 0x100
tcp equ 0x61    ; NTCPDRV interrupt
    xor ax,ax
    mov bx,10
    xor cx,cx
    mov si,0x81     ; [ds:81]-[ds:FF] = command line args
    mov di,0x80     ; [ds:80] = strlen(args)
    mov cl,[di]
    add di,cx
@@: inc si
    mov cl,[si]     ; get character
    sub cl,'0'      ; convert char to int
    mul bx          ; ax *= 10
    add al,cl
    cmp si,di
    jb @b
    ; now ax = port number
    mov bx,ax       ; source port (leaving this 0 doesn't work?)
    mov cx,ax       ; dest port
    mov ax,0x1001   ; open TCP socket for listening
    mov dx,-1       ; infinite timeout
    xor si,si       ; any dest IP
    xor di,di
    int tcp
    ; ^ I think this call should block until a connection is established, but apparently it doesn't.
    push bx         ; bx = socket handle, save it for later
    mov ax,0x1200   ; read from socket
    mov di,0x80     ; es:di = buffer (just reuse argument area to save space)
    mov cx,1        ; one byte
    mov dx,-1
    int tcp         ; this will block until a client connects and sends one byte
    mov ax,0x1400   ; get TCP session status, bx=handle
    int tcp
    ; now es:di points to a struct containing the source/dest IP addresses and ports
    ; the docs say it's two dwords for each IP address, then two bytes for "ip_prot" and "active" (whatever that means)
    ; ...but actually each IP address is followed by the port number (one word)
    xor ax,ax
    mov bx,10
    add di,6        ; [es:di+6] = client IP
    lea cx,[di+4]
@@: add al,[es:di]  ; add all bytes together
    adc ah,0
    inc di
    cmp di,cx
    jb @b
    ; now ax contains the IP address sum
    mov di,0x84     ; recycle arguments area again
    mov cx,0x80
    mov bx,10
@@: dec di
    xor dx,dx
    div bx          ; dl = ax mod 10
    add dl,'0'      ; convert int to char
    mov [di],dl
    cmp di,cx
    ja @b
    ; now [ds:80]-[ds:83] contains an ascii representation of the IP address sum
    push ds
    pop es
    mov ax,0x130e   ; send data with newline, wait for ack
    pop bx          ; socket handle
    mov di,0x80     ; es:di = data
    mov cx,4        ; sizeof data
    mov dx,-1
    int tcp
    mov ax,0x1100   ; close TCP socket
    mov dx,1
    int tcp
    mov ax,0x4c00
    int 0x21
```
This assumes [`ntcpdrv`](http://www.pld.ttu.ee/~priidu/library/net/ntcpdrv.zip) is loaded at `INT 0x61` (and any suitable packet driver at `0x60`). Compile with `fasm tcpserv.asm`.
It has some issues though:
* It doesn't check if the argument is a valid port number, or if it's even a number at all.
* The client must send at least one byte, since I can't seem to find any other way to tell if a client has connected.
* It only works once, and hangs on a second attempt. Works again after a reboot.
* The returned value is left-padded with zeroes.
* This is my very first code golf entry, and also my very first 8086 asm program. I'm sure there are ways to improve this further.
[Answer]
# LiveScript, ~~107~~ 105 bytes
```
(require \http)createServer(->&1.end((.reduce (+))<|it.connection.remoteAddress/\.))listen process.argv.0
```
Nothing much to add, it's just basic NodeJS stuff. Style points for `&1` (second argument), `<|` (F# piping, akin to `$` in Haskell) and biop: `(+)` in LS is like operator sections in Haskell: a curried binary function (that adds its operands).
Also a bit dirty: `/`, if given a *literal* string on its right, will do split.
[Answer]
# Perl, ~~141~~ 132 + 1 = 133 bytes
## Golfed
```
$s=new IO::Socket::INET LocalPort=><>,Listen=>5,Reuse=>1;{$c=$s->accept;$_=$c->peerhost;y/./+/;$c->send(eval.$/);shutdown $c,1;redo}
```
## Ungolfed
```
# listen on tcp port obtained from stdin
$s=new IO::Socket::INET(LocalPort=> <>,
                        Listen   => 5,
                        Reuse    => 1);
{
    # accept connection
    $c=$s->accept();
    
    # get the ip address
    $_=$c->peerhost();
    # replace dots with plus
    y/./+/;
    # send the evaluated version back, with a newline
    $c->send(eval . $/);
    # close
    shutdown($c,1);
    redo;
}
```
## Example
```
$ echo 7777|perl -MIO::Socket::INET -e'$s=new IO::Socket::INET LocalPort=><>,Listen=>5,Reuse=>1;{$c=$s->accept;$_=$c->peerhost;y/./+/;$c->send(eval.$/);shutdown $c,1;redo}'
$ telnet 127.0.0.1 7777
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
128
Connection closed by foreign host.
$
```
[Answer]
# Python 2, 180 bytes
```
from SocketServer import*
TCPServer(('',input()),type('',(BaseRequestHandler,set),{'handle':lambda s:s.request.send(`eval(s.client_address[0].replace('.','+'))`)})).serve_forever()
```
Takes the port over stdin.
[Answer]
# NodeJS (ES6), ~~129~~ ~~118~~ 107 bytes
```
require('net').createServer(c=>c.end(eval(c.remoteAddress.replace(/\./g,'+'))+`
`)).listen(process.argv[2])
```
Works for IPv4. Run as `node server.js <port>`
[Answer]
## Go, 211 bytes
```
package main
import(."fmt"
."net"
"os")
func main(){s,_:=Listen("tcp4",":"+os.Args[1])
for{c,_:=s.Accept()
var a,b,d,e int
Sscanf(Sprint(c.RemoteAddr()),"%d.%d.%d.%d",&a,&b,&d,&e)
Fprintln(c,a+b+d+e)
c.Close()}}
```
Can probably be golfed further, I'm not entirely satisfied with the way I have to parse the IP adress for instance, it looks like a horrible hack.
Listens on IPv4 on the port given as argument.
[Answer]
## PowerShell, 208 206 192 152 bytes
```
($t=[net.sockets.tcplistener]$args[0]).start();for(){($z=$t.acceptsocket()).sen‌d([char[]]"$($z.remoteendpoint.address-replace"\.","+"|iex)");$z.close()}
```
version information:
```
Name                           Value
----                           -----
PSVersion                      4.0
WSManStackVersion              3.0
SerializationVersion           1.1.0.1
CLRVersion                     4.0.30319.34209
BuildVersion                   6.3.9600.17400
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0}
PSRemotingProtocolVersion      2.2
```
Thanks to TimmyD for saving me 14 bytes!
Massive thanks to TessellatingHeckler for saving me 40 bytes
[Answer]
## C, 535 bytes
Well, someone had to do this.
I added a single line break so the posted code actually has 536 characters.
```
#include <stdio.h>
#include <netdb.h>
#include <netinet/in.h>
#include <string.h>
int main(int c,char**v){int f,l;char b[99];struct sockaddr_in d,e;f=socket(AF_INET,SOCK_STREAM,0);bzero(&d,sizeof(d));d.sin_family=AF_INET;d.sin_addr.s_addr=INADDR_ANY;d.sin_port=htons(atoi(v[1]));bind(f,&d, sizeof(d));listen(f,5);l=sizeof(e);
f=accept(f,&e,&l);bzero(b,99);int p,q,r,s;char g[INET_ADDRSTRLEN];inet_ntop(AF_INET,&(e.sin_addr),g,INET_ADDRSTRLEN);sscanf(g,"%d.%d.%d.%d",&p,&q,&r,&s);sprintf(b,"%d\n",p+q+r+s);write(f,b,strlen(b));return 0;}
```
compile with `gcc [file_name] -o server`
run with `./server [port]`
connect with `telnet localhost [port]`
[Answer]
# Haskell, 216 bytes
Using the "network-simple" package (`cabal install network-simple`). Needs a couple of language extensions (`-XOverloadedStrings -XNoMonomorphismRestriction`) to work.
```
import Network.Simple.TCP(serve)
import Network.Socket
import Data.Bits
main=getLine>>= \n->serve"*"n p
p(s,(SockAddrInet _ h))=()<$(send s$(show$sum$w h 24)++"\n")
m=255
w h 0=[h.&.m]
w h n=h`shiftR`n.&.m:(w h$n-8)
```
There are a couple of possible simplifications, including changing the `w` function to return the sum directly rather than a list, and using a function rather than a program so that the port number can be specified as an argument. I don't imagine this would reduce the size very much, though. 20 bytes perhaps?
[Answer]
# Mumps, ~~114~~ 115 Bytes
Golfed:
```
R P F{S J=0,I="|TCP|1" O I:(:P) U I R K F K=1:1:4{S J=J+$P(##class(%SYSTEM.TCPDevice).PeerAddr(),".",K)} W J,! C I}
```
Ungolfed:
```
R P             ; Read Port # from STDIN ;
  F{            ; Loop over everything;
  S J=0,        ; Initial IP segment total
  I="|TCP|1"    ; TCP device
  O I:(:P)      ; Open the TCP device, port from input {and sticking a tongue out! :-) }
  U I           ; Use the TCP device
  R K           ; Read from STDIN (anything)
  F K=1:1:4{    ; Iterate 1->4 in variable K
    S J=J+      ; Accumulate the following segments of the IP in var. J
    $P(##class(%SYSTEM.TCPDevice).PeerAddr(),".",K) ; Grab each piece of IPv4.
            }   ; close the loop.
  W J,!         ; Write the total w/newline out the TCP port 
  C I           ; close the TCP port to send.
}               ; end final loop
```
This is the InterSystems Caché version of Mumps - if there's a version out there that can acquire the TCP address shorter than `##class(%SYSTEM.TCPDevice).PeerAddr()` *(as it's almost a 1/3 of the entire program)* it might have a better chance against some of the other languages posted already... ;-)
Edit: Thanks to @TimmyD - I missed the reading of the port from STDIN or arguments instead of being hardcoded. Edited; it added 1 byte to the program.
[Answer]
# C, 243 188 bytes (or perhaps 217 162 bytes)
V2: see below for the explanations.
188 bytes:
```
s;main(g,v)char**v;{short S[8]={2,htons(atoi(v[1]))};char C[g=16];bind(s=socket(2,1,0),&S,g);for(listen(s,8);g=fdopen(accept(s,&C,&g),"w");fclose(g))fprintf(g,"%d\n",C[4]+C[5]+C[6]+C[7]);}
```
Slightly circumspect 162 bytes:
```
s;main(g){short S[8]={2,g};char C[g=16];bind(s=socket(2,1,0),&S,g);for(listen(s,8);g=fdopen(accept(s,&C,&g),"w");fclose(g))fprintf(g,"%d\n",C[4]+C[5]+C[6]+C[7]);}
```
Probably more golfing possible tomorrow morning. I'll tidy this post up after those updates.
---
V1:
This one was really quite fun to golf. 
```
#include<netdb.h>
s,c,q;main(g,v)char**v;{struct sockaddr_in S={2,htons(atoi(v[1]))},C;bind(s=socket(2,1,0),&S,g=16);for(listen(s,8);c=accept(s,&C,&g);q=fclose(g)){for(g=4;g;q+=C.sin_addr.s_addr>>8*--g&255);fprintf(g=fdopen(c,"w"),"%d\n",q);}}
```
It works for IPv4. Mostly it's a straightforward implementation. The three main components are
Creating the socket:
```
struct sockaddr_in S={2,htons(atoi(v[1]))},C;bind(s=socket(2,1,0),&S,g=16);
```
We use the various explicit forms of the constants AF\_INET etc, and make use of the fact that when a struct is initialised in C in this way the non-specified elements are set to zero. 
Listen for clients, accept them, and close their connections:
```
for(listen(s,8);c=accept(s,&C,&g);q=fclose(g))
```
Finally to send each client the data:
```
for(g=4;g;q+=C.sin_addr.s_addr>>8*--g&255);fprintf(g=fdopen(c,"w"),"%d\n",q);
```
The IP is stored in `C.sin_addr.s_addr` as a 32 bit integer where each octet is represented by one of the four bytes. We sum these bytes with the for loop and then print them to the stream using fprintf. 
I have a shorter 217 byte solution but I'm not entirely sure it doesn't violate the standard loopholes since it requires that the port is given in unary in network byte order as command line arguments. That is, to run the server on port 12345 one would need to call
```
$ ./tcp 1 1 1 1 ... 1 1 1
```
where the total number of `1`s is 14640. To say the least it's a little... cumbersome. But here it is anyway:
```
#include<netdb.h>
s,c,q;main(g){struct sockaddr_in S={2,g},C;bind(s=socket(2,1,0),&S,g=16);for(listen(s,8);c=accept(s,&C,&g);q=fclose(g)){for(g=4;g;q+=C.sin_addr.s_addr>>8*--g&255);fprintf(g=fdopen(c,"w"),"%d\n",q);}}
```
[Answer]
# Java, 210 bytes
Golfed:
```
p->{java.net.ServerSocket x=new java.net.ServerSocket(p);for(;;){try(java.net.Socket s=x.accept()){byte[]b=s.getInetAddress().getAddress();s.getOutputStream().write((0+b[0]+b[1]+b[2]+b[3]+"\n").getBytes());}}};
```
Expanded:
```
@FunctionalInterface interface Consumer { // Define an interface that allows a function that throws an exception.
  void accept(int port) throws Exception;
}
Consumer consumer = (port) -> {
  java.net.ServerSocket serverSocket = new java.net.ServerSocket(port);
    for (;;) {
      try (java.net.Socket socket = serverSocket.accept()) {
        byte[] bytes = socket.getInetAddress().getAddress();
        socket.getOutputStream().write((0 + b[0] + b[1] + b[2] + b[3] + "\n").getBytes());
      }
    }
}
```
This a gathering of all the tips I gave in other Java answers, plus the writing as a function instead of a full program, which gains roughly 70 bytes compared to the program.
[Answer]
# Haskell, 326 bytes
```
import Data.Bits
import Network.Socket
import System.IO
f n=withSocketsDo$do
 s<-socket AF_INET Stream defaultProtocol
 bind s$SockAddrInet n iNADDR_ANY
 listen s 1
 g s
g s=do
 (z,SockAddrInet _ a)<-accept s
 h<-socketToHandle z WriteMode
 hPutStrLn h$show$b a
 hClose h
 g s
b 0=0
b x=x.&.0xFF+b(x`shiftR`8)
```
Sadly I had to use `Network.Socket` to get access to the remote IP address as an integer rather than a string. It would have saved dozens of characters if I could just do `s <- listenOn (PortNumber n)`, rather than having to explicitly call `socket`, `bind` and `listen` individually. But, sadly, `Network.accept` gives me a host *string*, not an IP address *integer*, so I had to resort to `Network.Socket.accept` and friends.
The function `f` takes a port number as argument, and creates a server socket (`s`) listening on that port. It then calls the function `g` with the server socket. `g` loops forever, accepting connections. The function `b` takes an actual IPv4 address and computes the sum of its digits.
I'm sure somebody somewhere can do this better than me. I wanted to show off just how damned easy socket stuff is in Haskell... but then failed miserably, because I need access to the IP address, which isn't usually easy to get.
[Answer]
# Lua, ~~169~~ ~~162~~ ~~160~~ ~~153~~ ~~151~~ ~~148~~ ~~138~~ 129 bytes
**Golfed version**
```
m=require"socket".bind(0,...)::l::c=m:accept()f=0 for n in c:getpeername():gmatch"%d+"do f=f+n end c:send(f.."\n")c:close()goto l
```
It requires Luasocket to be installed and an interpreter that supports labels. I've tested it with Luajit and I can also confirm that the code does not work with Lua 5.1.
**Ungolfed version**
```
m=require"socket".bind(0,...)
::l::
c=m:accept()
f=0
for n in c:getpeername():gmatch"%d+" do
    f=f+n
end
c:send(f.."\n")
c:close()
goto l
```
**Edit 1:**
Changed `i=({c:getpeername()})[1]` to just `i=c:getpeername()`
**Edit 2:**
Removed braces from the require statement.
**Edit 3:**
Removed the braces around the vararg, decreased the byte count a little bit.
**Edit 4:**
Removed the parenthesis around "%d+", shorter by 2 bytes.
**Edit 5:**
Removed the unnecessary variable i.
**Edit 6:**
Changed the ip from "127.0.0.1" to 0. (Thanks to xyzzy on #lua)
**Edit 7:**
Removed the function call to tonumber since strings are cast to numbers automatically (Thanks to Trebuchette for the suggestion, I did not know this)
[Answer]
# Haskell, 185 (+ 19 = 204)? bytes
```
import Network.Simple.TCP(serve)
import Network.Socket
import Data.List.Split
main=getLine>>=flip(serve"*4")(\(a,b)->()<$(send a$(++"\n")$show$sum.map read.take 4.sepByOneOf":."$show b)
```
Takes port number as one line on stdin; requires `network-simple` from Cabal.
As usual with Haskell answers that don't constrain themselves to pure functions, the `imports` take up far too many bytes. The trailing newline is also worth 9 bytes...
Somewhat similar to @Jules's answer, but I use string manipulation instead of byte operations. I ~~stole~~ used the `-XOverloadedStrings` extension as well, which is probably worth 19 bytes extra.
[Answer]
# Racket, 265 bytes
```
#lang racket(define l(tcp-listen(string->number(read-line))))(let e()(define-values(i o)(tcp-accept l))(thread(λ()(define-values(a b)(tcp-addresses o))(write(~a(foldl + 0(map string->number(string-split a".")))o))(newline o)(close-output-port o)))(e)))
```
Ungolfed:
```
#lang racket
(define listener (tcp-listen (string->number (read-line))))
(define (mk-server)
  (let echo-server ()
    (define-values (in out) (tcp-accept listener))
    (thread
     (λ()
       (define-values (a b) (tcp-addresses out))
       (write (number->string (foldl + 0(map string->number(string-split a "."))) out))
       (write "\n" out)
       (close-output-port out)))
    (echo-server)))
```
[Answer]
# Factor, ~~155~~ ~~146~~ ~~131~~ ~~206~~ 190 bytes
Well, I just learned a lot about low-level socket programming. I don't think I ever wanna do that again, because my ~~thr~~head hurts.
```
[ utf8 <threaded-server> readln 10 base> >>insecure [ remote-address get host>> "." split [ 10 base> ] map sum 10 >base print flush ] >>handler [ start-server ] in-thread start-server drop ]
```
Oh yeah, threaded, and doesn't return, right.
]