IP Configuration

Understanding and managing IP addresses is the foundation of networking on Unix and Linux systems. In this section, we’ll focus on viewing interfaces, checking IP addresses and assigning temporary addresses for testing.

Viewing Network interfaces

The modern command to inspect interfaces is ip (Preferred over the older ifconfig)

shellsession 
user@machine:/$ ip link show
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
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
  link/ether 00:15:5d:4b:89:84 brd ff:ff:ff:ff:ff:ff

Note that eth0 can vary some times. Be sure to check that your interface is eth0. If not, use whatever thats there.

Checking IP addresses

To see assigned IPs, use ip addr(can be shortened to ip a)

shellsession 
user@machine:/$ ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
  link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
  inet 127.0.0.1/8 scope host lo
     valid_lft forever preferred_lft forever
  inet 10.255.255.254/32 brd 10.255.255.254 scope global lo
     valid_lft forever preferred_lft forever
  inet6 ::1/128 scope host proto kernel_lo
     valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
  link/ether 00:15:5d:4b:89:84 brd ff:ff:ff:ff:ff:ff
  inet 172.29.26.129/20 brd 172.29.31.255 scope global eth0
     valid_lft forever preferred_lft forever
  inet6 fe80::215:5dff:fe4b:8984/64 scope link proto kernel_ll
     valid_lft forever preferred_lft forever

The highlighted line shows the IPv4 address of eth0, 172.29.26.129/20, which has the broadcast address(brd) of 172.29.31.255. Scope refers to the range of visibility (global vs link-local). Since inet refers to IPv4, inet6 refers to IPv6.

Assigning IP Addresses Temporarily

You can assign an IP to an interface (will reset on boot) using ip addr add. This requires root or sudo permissions.

shellsession 
user@machine:/$ sudo ip addr add 192.168.1.50/24 dev eth0

sudo ip link set eth0 up

Ensure your network interface is UP.

shellsession 
user@machine:/$ sudo ip addr del 192.168.1.50/24 dev eth0

You will need to use ip a to verify changes as these commands do not print any output.

Viewing Routes

Check the routing table to see how packets are forwarded via ip route show.

shellsession 
user@machine:/$ ip route show
default via 172.29.16.1 dev eth0 proto kernel
172.29.16.0/20 dev eth0 proto kernel scope link src 172.29.26.129

Quick Reference

shellsession 
user@machine:/$ ip link show # list interfaces 
user@machine:/$ ip addr show # show IP addresses 
user@machine:/$ sudo ip addr add # assign IP temporarily 
user@machine:/$ sudo ip addr del # remove IP 
user@machine:/$ ip route show # view routing table