A Problem in Pigeonhole Principles. 
A computer network consists of six computers. Each computer is
  directly connected to at least one of the other computers. Show that
  there are at least two computers in the network that are directly
  connected to the same number of other computers.

I want to solve this problem with pigeonhole principle, but I can't do it.
Please give me a hint or answer.

Note: the most computers any one computer connect to is 5.  There just aren't any other computers to connect to.
So you have 5 possible numbers of connections, and 6 computers so... pigeon hole!
