ip Command in Linux: Addresses, Links, Routes, and Troubleshooting

Start every network check by seeing which interfaces exist and which addresses they hold.

Run the command:

ip -br addr show

Sample output:

lo               UNKNOWN        127.0.0.1/8 ::1/128 
enp0s3           UP             10.0.2.15/24 fd17:625c:f037:2:…/64 … 
enp0s8           UP             192.168.0.4/24 fe80::…/64

The first column is the interface name; UP means the link is active. Use ip addr show dev IFACE when you need prefix length, scope, and secondary addresses.

Link state must be UP before addresses on that interface are usable.

Run the command:

ip link show enp0s3

Sample output:

2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 08:00:27:26:59:37 brd ff:ff:ff:ff:ff:ff
    altname enx080027265937

LOWER_UP means the cable or virtual link has carrier. If you see state DOWN, bring the interface up before adding routes through it.

ip addr add is the standard way to assign a temporary address — safe to demo on lo in a lab.

Add a secondary loopback address:

sudo ip addr add 127.0.0.2/8 dev lo
ip -4 addr show dev lo

Sample output:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet 127.0.0.2/8 scope host secondary lo
       valid_lft forever preferred_lft forever

Revert when done:

sudo ip addr del 127.0.0.2/8 dev lo

Production changes on real NICs need matching subnet planning and often a persistent config layer (netplan, nmcli).

ip route get shows the interface, gateway, and source address for a destination without sending packets.

Run the command:

ip route get 8.8.8.8

Sample output:

8.8.8.8 via 10.0.2.2 dev enp0s3 src 10.0.2.15 uid 0 
    cache

Use this when ping works to one subnet but not another — it tells you whether policy routing or a missing default gateway is the issue.

Rescue shells and container hosts sometimes need you to toggle link state manually.

Run the commands on loopback (lab only — do not leave lo down on a production box):

sudo ip link set lo down
ip link show lo | head -1
sudo ip link set lo up
ip link show lo | head -1

Sample output:

1: lo: <LOOPBACK> mtu 65536 qdisc noqueue state DOWN mode DEFAULT group default qlen 1000
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000

On physical NICs use the real name from ip link (enp0s3, eth0, …) instead of lo.

Static routes send traffic for a prefix through a specific gateway — common in split-tunnel VPN lab setups.

Add a test route:

sudo ip route add 192.168.99.0/24 via 10.0.2.2 dev enp0s3
ip route show 192.168.99.0/24

Sample output:

192.168.99.0/24 via 10.0.2.2 dev enp0s3

Remove it:

sudo ip route del 192.168.99.0/24

If via is unreachable, the kernel may reject the route — confirm gateway reachability with ip route get GATEWAY_IP.

Counters help spot dropped packets or a stuck interface.

Run the command:

ip -s link show lo

Sample output:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    RX:  bytes packets errors dropped  missed   mcast           
       4614855   14920      0       0       0       0 
    TX:  bytes packets errors dropped carrier collsns           
       4614855   14920      0       0       0       0

Rising errors or dropped on a physical NIC points to cable, driver, or ring-buffer issues — not just IP misconfiguration.

Adding the same address twice returns an error — unlike ip route replace, add does not overwrite.

Run twice:

sudo ip addr add 127.0.0.2/8 dev lo
sudo ip addr add 127.0.0.2/8 dev lo

Sample output on the second run:

Error: ipv4: Address already assigned.

Use ip addr del first, or ip addr replace when you intend to swap prefixes on an interface.

Before adding routes in rescue mode, snapshot what the kernel already knows.

Run the command:

ip route show

Sample output (truncated):

default via 10.0.2.2 dev enp0s3 proto dhcp src 10.0.2.15 metric 100 
10.0.2.0/24 dev enp0s3 proto kernel scope link src 10.0.2.15 metric 100 
192.168.0.0/24 dev enp0s8 proto kernel scope link src 192.168.0.4 metric 101

proto dhcp means NetworkManager or systemd-networkd installed the route — your manual ip route changes may disappear on DHCP renew unless made persistent elsewhere.