The LAN assumptions programmers should stop making

Dreamstation Systems has published “Falsehoods Programmers Believe About LANs”, a technical guide to assumptions about local area networks that may hold in some environments but are not universally reliable.

The article, surfaced via Hacker News, covers a broad range of networking topics, including network address translation, protocols, MAC addresses, reachability and performance.

Its cautions include assumptions that a LAN is behind one NAT, that private IP addresses reveal whether NAT is in use, and that only one protocol or traffic type is operating on a network.

The list also questions common beliefs about MAC addresses, including whether they are globally or locally unique, whether different addresses identify different devices, and whether they can be changed.

Other entries address practical network behaviour. They challenge assumptions that a LAN is always fast, that hosts on the same subnet can communicate directly, or that Wi-Fi and Ethernet clients are equivalent peers.

The article also examines DHCP and IP addressing, including assumptions about the number of DHCP servers, whether a lease will remain unchanged, what configuration DHCP will provide, and whether every host has a single IP address.