Beginner · Linux Network Connectivity · section 2 of 11

Interfaces and addresses

✓ Lab-tested 17 October 2026 on Debian 12

Modern Linux uses the ip command from iproute2 for everything network-related. ifconfig, route and arp come from the long-deprecated net-tools package and are often not even installed.

ip link shows interfaces: name, state and MAC address. It's layer 2.
ip addr shows addresses: IPv4/IPv6 addresses and prefix lengths. It's layer 3.
Add -br (brief) for a one-line-per-interface summary, and -c for colour.

What to look for:

State UP with LOWER_UP in the flags means the interface is enabled and has a carrier (cable/link). UP without LOWER_UP means it's enabled but nothing is plugged in. DOWN means an admin turned it off (ip link set eth0 up).
The prefix length matters as much as the address. 192.0.2.10/24 and 192.0.2.10/32 behave completely differently: the prefix decides which other addresses are "on the same LAN".
lo is the loopback interface, always present.

Changes made with ip are immediate and not persistent: they vanish on reboot. Permanent configuration lives in your distribution's network manager (netplan on Ubuntu, NetworkManager on Fedora/RHEL, systemd-networkd, or /etc/network/interfaces on Debian); see the last section.

The client's interfaces (real output)

root@client:~# ip -br link
lo               UNKNOWN        00:00:00:00:00:00 <LOOPBACK,UP,LOWER_UP> 
eth0@if123       UP             1a:dc:74:6a:45:dc <BROADCAST,MULTICAST,UP,LOWER_UP> 
root@client:~# ip -br addr
lo               UNKNOWN        127.0.0.1/8 
eth0@if123       UP             192.0.2.10/24 
root@client:~# ip addr show dev eth0
2: eth0@if123: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default 
    link/ether 1a:dc:74:6a:45:dc brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 192.0.2.10/24 scope global eth0
       valid_lft forever preferred_lft forever

Lines 1–3: Layer 2: interface names, state and MAC addresses. @if123 means this is one end of a virtual ethernet (veth) pair whose peer is interface 123 in another namespace, as in containers.
Lines 4–6: Layer 3: one line per interface with its addresses. The fastest health check there is.
Line 8: Flags: UP (enabled) and LOWER_UP (carrier present). The MTU is 1500 bytes, standard Ethernet.
Line 9: The MAC address the router will learn via ARP.
Line 10: The address and the prefix: /24 makes 192.0.2.0–255 "on-link". scope global means it's usable for traffic leaving the host.
Line 11: Lifetimes are forever for static addresses. DHCP-assigned addresses count down to the lease expiry.

Check your understanding

ip -br link shows eth0 UP … <NO-CARRIER,BROADCAST,MULTICAST,UP>. What's wrong?

The interface is administratively down
There's no physical link: cable, switch port or Wi-Fi association
It has no IP address
The MTU is wrong