remote_address

TCPSocket.open("www.ruby-lang.org", 80) {|s|
  p s.remote_address #=> #<Addrinfo: 221.186.184.68:80 TCP>
}

TCPServer.open("127.0.0.1", 1728) {|serv|
  c = TCPSocket.new("127.0.0.1", 1728)
  s = serv.accept
  p s.remote_address #=> #<Addrinfo: 127.0.0.1:36504 TCP>
}