ip addr & ip route

Your address and your route

Lesson 18 of 20 · 10 min · Module 6: Networking from the shell

Every networked computer has interfaces, addresses, and a route to the rest of the world. The ip command shows all three.

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By the end, you’ll be able to…

List interfaces and addresses
Recognise loopback and local addresses
Find the default gateway

The idea

ip addr (or ip a) lists network interfaces and their addresses. lo is the loopback interface (127.0.0.1, the machine talking to itself). ip route shows where traffic goes. The default via line names the gateway: the router for every address not on your local network.

ip addr show eth0
ip route

01Read an interface block

inet 192.168.1.50/24 is the IPv4 address with its prefix length: the first 24 bits (192.168.1) identify the local network. link/ether is the hardware (MAC) address of the network card.

ip addr show eth0

Output

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP
    link/ether 52:54:00:12:34:56
    inet 192.168.1.50/24 scope global eth0

02Loopback never leaves the machine

127.0.0.1 on lo always means "this computer". Services that listen only on 127.0.0.1 cannot be reached from other machines, which is often deliberate for databases.

03The default route is the way out

192.168.1.0/24 dev eth0 means local addresses are reached directly. Everything else follows default via 192.168.1.1, the router.

Your turn

Show the details of the eth0 interface only. Then save the default route line from ip route into route.txt in your home folder.

What the checks look for

Shows eth0 with address 192.168.1.50/24
route.txt contains only the default route

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Take this with you

ip addr shows who you are on the network; ip route shows how you reach everyone else.

Go a little further

Run ip route and explain in one sentence why 192.168.1.20 does not use the gateway.