This post is for small office use / for beginners.
Many would require to use Linux as Internet gateway. Connecting DSL links over Linux server you can filter URL & get proxy caching to reduce link consumption, use DHCP server, use internal DNS server, create internet usage report & block unwanted traffic using iptables.
Configure DHCP to avoid ip address conflicts.
Use your own DNS for internal / external world.
You can use squid proxy over gateway for caching & url filtering.