1] Get a 24 Port Switch
2] Crimp 20 LAN Cables according the length requorement [keep crimping straigh and not cross over]
3] connect all the cables from LAN ports on PCs to switch
4] in the onboard LAN properties,go under TCP/IP
5] Enter IP address for 20 PCS from 192.168.0.1 - 192.168.0.21
6] Keep 192.168.0.1 for Server
7] Keep subnet mask - 255.255.255.0 for all PCs
8] Apply settings
9] Go to control panel--> performance and maintainence---> system---> computer name
10] Enter whatever description you want in the Computer description area in each PC
11] click on change tab
12] enter computer name as per your liking
13] enter workgroup as per choice.
14] remember to keep workgroup same for all PCs
youa re done with basic networking.