Changing IP Addresses and Routes in Linux

Once the machine is booted and connected to the Ethernet, it's ready for IP reconfiguration

ifconfig and route output before the change

[root@morgan]# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 00:80:C8:F8:4A:53
inet addr:192.168.98.82 Bcast:192.168.98.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)
Interrupt:9 Base address:0x5000

[root@morgan]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.98.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
0.0.0.0 192.168.98.254 0.0.0.0 UG 0 0 0 eth0


Bringing down a network interface with ifconfig

[root@morgan]# ifconfig eth0 down

This is a fast way to stop networking on a single-homed machine such as a server or workstation. On multi-homed hosts, other interfaces on the machine would be unaffected by this command. This method of bringing down an interface has some serious side effects, which should be understood. Here is a summary of the side effects of bringing down an interface

Side effects of bringing down an interface with ifconfig

all IP addresses on the specified interface are deactivated and removed

any connections established to or from IPs on the specified interface are broken [7]

all routes to any destinations through the specified interface are removed from the routing tables

the link layer device is deactivated

Bringing up an Ethernet interface with ifconfig

[root@morgan]# ifconfig eth0 192.168.99.14 netmask 255.255.255.0 up

[root@morgan]# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 00:80:C8:F8:4A:53
inet addr:192.168.99.14 Bcast:192.168.99.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)
Interrupt:9 Base address:0x5000

The second call to ifconfig allows verification of the IP addressing information. The currently configured IP address on eth0 is 192.168.99.14.