Routing Concept

Route Table

For a network router to know where to send packets of data it receives

Main entries if route table :

Destination IP address : Represents the IP address of the destination.
Gateway : The IP address of the next router which decides how to further send an IP data gram received on its interface.
Flags : Provides another set of vital information like destination IP address is a host address or a network address. Also, flags convey whether the Gateway is really a next router or a directly connected interface.
Network interface specs : Some specification about the network interface the datagram should be passed for further transmission.
Metric : The Metric field indicates the cost of a route. If multiple routes exist to a given destination network ID, the metric is used to decide which route is to be taken. The route with the lowest metric is the preferred route.

'route' Command Usage

refer: https://linoxide.com/how-tos/add-static-route-linux/

(1) Show route table

$ route -n

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         172.31.0.1      0.0.0.0         UG    0      0        0 eth0
169.254.169.254 0.0.0.0         255.255.255.255 UH    0      0        0 eth0
172.31.0.0      0.0.0.0         255.255.240.0   U     0      0        0 eth0

-n option, which will only show numerical addresses rather than performing any sort of name resolution.
The flags:
- U: suggests that the route is up
- G: suggests that the router is to a gateway (router).
- H: this is a route to a single host (and not a network)

Note: Other command to get route table:
$ netstat -an
$ ip route

(2) Adds a route to the network

route add -net \ gw \ \

to indicate the destination network we want to join. We must specify the mask of the network and the gateway to use.

route add -net 10.0.0.0/8 gw 192.168.1.1 eth1

route add -host \ gw \

to indicate a specific host that we want to join

route add -host 10.0.0.10 gw 192.168.1.1 eth1

Add Default route:

route add default gw 192.168.1.10

(3) Deleting a route rule

route del -net \ gw \ \

to delete a destination network from the routing table

(4) Reject a route

route add -host \ reject

to ban an IP address but it is still on the routing table.

Route