DHCP: the address the network hands you, and what 169.254 means

A machine usually does not know its own address; it asks, and a DHCP server answers with an address, a mask, a gateway and a DNS server, on loan for a lease time. An address beginning 169.254 is the machine's own fallback: it asked, and nobody answered.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

'No internet' on one desktop, and the address reads 169.254.23.9. That number is a sentence: 'I asked for an address and heard nothing.' It is not a broken internet, a wrong password or a firewall. It is the machine and the DHCP server failing to meet, and the checks are the wire between them and the server itself.

How to think about it

Read the address before anything else. 169.254.x.x: no lease; check the cable and link light, then release and renew, then try another port or another device on the same port to split the fault between this machine and the network. A real address with a lease: DHCP worked, and the fault is higher up. A real address that is wrong for the floor: the machine reached the wrong DHCP server, or a static one was typed in.

Worked example

Address 169.254.23.9, no gateway, no DNS

Self-assigned. Nothing on the network gave this machine anything.

Link light lit → the cable is fine; the request is going out

So the fault is not the wire. The server, or the path to it.

Release, then renew → 10.4.2.31, mask 255.255.255.0, gateway 10.4.2.1, lease 8 days

A second ask got an answer. The first one was lost, or the server was busy, or the port had just come up.

ping 10.4.2.1 → reply

The gateway answers. The lease is real and the machine is on the network.

Your turn

A laptop shows 169.254.8.140. Write what it did not get.

169.254.8.140 → no  from DHCP

The trap

Typing a static address to 'fix' a 169.254. It works for that machine today, and next month it collides with the address DHCP hands to somebody else, and two people have a fault that comes and goes.

Practise DHCP and 169.254 on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.