ip addr

The first thing to run on any device: which cards it has, whether they are up, and the address with its prefix on each (10.0.1.10/24). The prefix decides which addresses count as "my own network": everything else goes to a router. ip -br addr is the one-line-per-card view. It replaced ifconfig, which you still meet in old guides.

ip addr command usage

ip [-br] [-4|-6] addr [show [dev CARD]]
ip addr add|del ADDRESS/PREFIX dev CARD
ip addr help

Options

-br

brief: one line per card, name, state, addresses

help

print every form the command takes, and stop

-4 / -6

only IPv4 or only IPv6 addresses

show dev eth0

only that card

add 10.0.1.50/24 dev eth0

put one more address on the card, with its prefix

del 10.0.1.50/24 dev eth0

take an address off the card, named with its prefix as the card lists it

scope global

skip link-local and loopback addresses

ip addr examples

ip -br addr

Lists every card on one line each, with its state and addresses.

ip addr show eth0

Shows eth0 in full: flags, MTU, MAC address, addresses with prefix and broadcast.

ip -6 addr show scope global

Shows only the global IPv6 addresses, the ones SLAAC or DHCPv6 gave.

ip addr add 10.0.1.50/24 dev eth0

Gives eth0 a second address, 10.0.1.50 in the 10.0.1.0/24 network.

ip addr del 10.0.1.50/24 dev eth0

Removes that address again.

ip -4 -br addr

Lists only the IPv4 addresses, one card per line.

ip -br -c addr

The same list in colour: states and address families stand out.

ip -br addr show up

Lists only the cards that are switched on.

ip addr show to 10.0.1.0/24

Finds which card holds an address in 10.0.1.0/24.

ip -6 addr show dev eth0 scope link

Shows eth0's link-local IPv6 address, the fe80:: one every IPv6 card has.

ip addr add 2001:db8:1::10/64 dev eth0

Gives eth0 a static IPv6 address.

ip addr flush dev eth0

Removes every address from eth0 at once.

ip -o -4 addr show eth0 | awk '{print $4}'

Prints eth0's IPv4 address and prefix alone, for a script: -o puts each address on one line.

ifconfig eth0

The old net-tools view of the same card, still found on older systems and in old guides.

Reading the output

$ ip addr show eth0
11: eth0@if6378: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default
    link/ether fe:26:a1:c3:bb:c3 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 10.0.1.10/24 brd 10.0.1.255 scope global eth0
       valid_lft forever preferred_lft forever

• <…UP,LOWER_UP>: UP means switched on, LOWER_UP means a cable (or its virtual twin) is connected. UP without LOWER_UP is a dead link.
• mtu 1500: the largest packet this card sends without fragmenting.
• link/ether fe:26:…: the card's MAC address.
• inet 10.0.1.10/24: the IPv4 address and prefix: this device treats 10.0.1.0–10.0.1.255 as its own network. brd is that network's broadcast address.
• @if6378: the card is one end of a virtual cable; the other end is interface 6378 on the host.

Pitfalls

• A wrong prefix is a classic fault: /16 where the network is /24 makes the device ARP for addresses that are really behind a router.
• ip addr add without /PREFIX means /32: an address with no network, and no route for its neighbours.
• Changes made with ip last until the device restarts: they are not saved anywhere.

Run it on real machines