ip addressing and routing

Level: - Beginner
Platform: - RED hat Linux 9.0
Keywords: - Assigning IP address, adding static routes, adding routes to startup
Author: - Dinesh Aggarwal

Assigning IP address to a network card is very simple. If you want to assign a IP address 172.31.117.1 with mask 255.255.255.0  to interface eth1 give the following command

ifconfig eth1 172.31.117.1 netmask 255.255.255.0

Now if you reboot the PC, this ip address will be gone.

NETWORK CONFIGURATION SHOULD BE SAVED IN THIS FILE
===============================================

vi /etc/sysconfig/network-scripts/ifcfg-eth0

In order to make IP address permanent, issue the following commands.

cd /etc/sysconfig/network-scripts

vi ifcfg-eth1

Now enter the following inside the file ifcfg-eth1 (Press I to start writing inside the file

DEVICE=eth1
ONBOOT=yes
BOOTPROTO=static
NETMASK=255.255.255.0
BROADCAST=172.31.117.1
IPADDR=172.31.117.1
NETWORK=172.31.117.0

(Press ESC and then  :w! )

Then file is saved now

Note:-
To save a vi file press esc then
:wq

To forcefully write a read only file
:wq!

To check the current IP address

root@localhost root]# ifconfig –a

eth0      Link encap:Ethernet  HWaddr 00:10:B5:CC:83:DB
BROADCAST MULTICAST  MTU:1500  Metric:1
RX packets:2058 errors:0 dropped:0 overruns:0 frame:0
TX packets:71 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:144622 (141.2 Kb)  TX bytes:7759 (7.5 Kb)
Interrupt:9 Base address:0x6f00

eth1      Link encap:Ethernet  HWaddr 00:D0:B7:BB:E6:2E
inet addr:172.31.117.1  Bcast:172.31.117.255  Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
RX packets:26071 errors:0 dropped:0 overruns:0 frame:0
TX packets:561 errors:6 dropped:0 overruns:0 carrier:6
collisions:2 txqueuelen:100
RX bytes:1499796 (1.4 Mb)  TX bytes:96543 (94.2 Kb)
Interrupt:5 Base address:0x74c0 Memory:feaff000-feaff038

lo        Link encap:Local Loopback
inet addr:127.0.0.1  Mask:255.0.0.0
UP LOOPBACK RUNNING  MTU:16436  Metric:1
RX packets:36914 errors:0 dropped:0 overruns:0 frame:0
TX packets:36914 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:2520187 (2.4 Mb)  TX bytes:2520187 (2.4 Mb)

Adding a static route:-

route  [-v]  [-A  family]  add [-net|-host] target [netmask Nm] [gw Gw]
[metric N] [mss M] [window W]  [irtt  I]  [reject]  [mod]  [dyn]
[reinstate] [[dev] If]

route add -net 192.56.76.0 netmask 255.255.255.0 dev eth1

       (adds a route to the network 192.56.76.x via "eth0". The Class  C
netmask modifier is not really necessary here because 192.* is a
Class C IP address. The word "dev" can be omitted here.)

Route add –net 192.168.1.0/24 gw 192.168.1.1

(Adds a route 192.168.1.0/255.255.255.255 and the gateway will be 192.168.1.1)

Route add –host 192.168.1.1/32 gw 192.156.1.1
( adds a route for one host/IP 192.168.1.1 and the packet destined for this host should go to 102.156.1.1)

Use the following command to check the present routes.

[root@localhost network-scripts]# netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
172.31.117.192  0.0.0.0         255.255.255.224 U         0 0          0 eth1
172.31.117.224  0.0.0.0         255.255.255.224 U         0 0          0 eth1
192.56.76.0  0.0.0.0         255.255.255.224 U         0 0          0 eth1
172.31.117.160  0.0.0.0         255.255.255.224 U         0 0          0 eth1
172.31.117.64   0.0.0.0         255.255.255.224 U         0 0          0 eth1
172.31.117.96   0.0.0.0         255.255.255.224 U         0 0          0 eth1
172.31.117.0    0.0.0.0         255.255.255.224 U         0 0          0 eth1
172.31.117.32   0.0.0.0         255.255.255.224 U         0 0          0 eth1
172.16.100.0    0.0.0.0         255.255.255.0   U         0 0          0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U         0 0          0 eth1
127.0.0.0       0.0.0.0         255.0.0.0       U         0 0          0 lo
0.0.0.0         172.16.100.1    0.0.0.0         UG        0 0          0 eth0
[root@localhost network-scripts]#

Adding routes to the startup:-

All these added routes will be lost whenever we restart the pc, in order to make these route entries permanent ot to make sure that these comes back automatically whenever a pc is restarted, issue the following commands.

Vi  /etc/rc.local

And insert the following command, save the file and exit.

route add -net 192.56.76.0 netmask 255.255.255.0 dev eth1
Route add –net 192.168.1.0/24 gw 192.168.1.1
Route add –host 192.168.1.1/32 gw 192.156.1.1

Rc.local is run every time your pc is restarted. 

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