Falsehoods Programmers Believe About LANs

2026-09-05

Each entry is phrased as the falsehood. Some of these may sometimes be true and sometimes be false.

NAT

• My LAN is behind a NAT.
• I’m only behind one NAT.
• My LAN isn’t behind a NAT.
• My IPv6 hosts aren’t behind a NAT.
• If I have a RFC 1918 address, I’m behind a NAT.
• If I don’t have a RFC 1918 address, I’m not behind a NAT.
• NAT only translates addresses.
• NAT only translates addresses and ports.
• NAT changes nothing outside of the header.
• NAT is only found on LANs.

Protocols

• Only IPv4 is running on my LAN.
• Only IP is running on my LAN.
• Okay, only IP, IPX/SPX, Phase IV, AppleTalk, VINES, NetBEUI, or SNA are running on my LAN.
• Seriously?
• I can stop certain types of traffic from using my LAN.
• I know what’s running on my LAN.

MAC addresses

• MAC addresses are globally unique. (They’re supposed to be, but they aren’t.)
• MAC addresses are locally unique.
• Different MACs are different devices.
• Okay, different MACs are different network interfaces.
• MAC addresses can’t be changed.
• MAC addresses can be changed.
• MAC addresses are 48 bits.
• The first three bytes are vendor‐unique.
• The first three bytes are vendor‐unique if it’s a MA-L block.
• MAC addresses are unicast.
• Only one host answers an ARP request for a given IP.
• ARP entries are accurate.

Reachability and performance

• The LAN is fast.
• The LAN is faster than the Internet.
• Two hosts on the same subnet can reach each other without leaving the LAN.
• Same subnet means two hosts can talk.
• Wi-Fi and Ethernet clients are equivalent peers on the LAN.
• Wi-Fi and Ethernet clients aren’t equivalent peers on the LAN.
• Every device on my LAN has the same MTU.
• Hostnames are locally unique.
• mDNS works.

DHCP

• There is a DHCP server.
• There is one DHCP server.
• My address won’t change while I’m using it.
• The DHCP server and the gateway are the same device.
• DHCP will give me a gateway.
• DHCP will give me a DNS server.
• 169.254.0.0/16 means DHCP failed.

IP addresses

• Every host has one IP address.
• My LAN uses RFC 1918 IP addresses.
• My LAN uses private IP addresses.
• My LAN uses IP addresses.