I think you're confused about network programming in general. In a post like this, the assumption is that the concurrency is happening over a single network interface. It doesn't matter really. The most conspicuous limiting factor on a POSIX-compliant system is the number of available file descriptors. When you listen() on an address and accept() a connection, the networking stack allocates a file descriptor for that connection. Then you can handle more connections.
http://pubs.opengroup.org/onlinepubs/9699919799/functions/ac...
http://pubs.opengroup.org/onlinepubs/9699919799/functions/li...
Beej's guide is a popular intro, but I'd bet any text on network programming covers sockets.