A Client and a Server have to agree on certain things before they start to communicate with each other. The first is the communications protocol they'll use. (The most common these days is TCP/IP. Ten years ago it was X.25 . IBM wishs the world used SNA, which was popular 20 years ago.)

Then you both have to agree how you'll exchange information using that communications protocol. Financial institutions now use something called FIX. The Stock Exchanges used to use proprietary message formats. Then you have to worry about the character set you communicate in...

So communications prgramming is very, very specific , very detail-oriented.