How to interpret /proc/pid/fd/n 'socket:'?

[Neil Bird <neil@xxxxxxxxxx>]

  When looking in /proc/pid/fd, when the fd is a socket does anyone 
know what the number there ("socket:[nnnn]") represents?

--
[neil@xxx ~]# rm -f .signature
[neil@xxx ~]# ls -l .signature
ls: .signature: No such file or directory
[neil@xxx ~]# exit