The role of routing tables and how to set up routing

I believe many friends have played with virtual machines and installed Linux systems in virtual machines. Generally, the first thing everyone does after installing a Linux system is to configure a static IP. To configure a static IP, in addition to configuring the IP and subnet mask, you also need to fill in the gateway address. After everything is configured, you will find that the host's routing table contains an entry for this gateway address. As for why the gateway address appears in the routing table, some friends may not be so clear. Now, let's take a look at what this routing table does.

The role of the routing table

The role of the routing table is actually similar to the role of navigation. It tells the host where the data packet should be forwarded. If the host does not have a routing table, then all its data packets cannot be sent out. So regardless of the router, the host will also have its own routing table.

Routing table format

Statement: All operations in this article are performed under a Linux host.

Checking the routing table is very simple. Just use route -n. The -n option means not to perform host reverse check.

# route -n 
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.110.1   0.0.0.0         UG    100    0        0 eth0
192.168.110.0   0.0.0.0         255.255.255.0   U     100    0        0 eth0

You can see that my host contains two routing tables. A piece of routing information mainly consists of several points:

• Destination address

• Next hop address

• Subnet mask

• Network card interface

Looking at the first route, the destination address is 0.0.0.0 and the subnet mask is also 0.0 .0.0, we call this route the default route. The default route is very important. It means that if no other route can forward the packet, then the default route will be used for forwarding. So, this is why the next hop address of the default route is the gateway address. Because my home network is relatively simple, it is only connected to one router. If I want to communicate with the external network, I must enter this router. The location of this router is also the gateway address. So, this explains the question at the beginning of the article.

Let’s look at the second route. It is also quite special. Its destination address is the network address of the local machine. The subnet mask is also the subnet mask of the local machine. Its next hop address is also the subnet mask of the local machine. It is 0.0.0.0. 0.0.0.0 means that it does not need to go through the router and is delivered directly. Then it means that to communicate with the host in the LAN, there is no need to enter the router, and it can be delivered directly.

Add routing table

It is also very simple to add a routing table. Use the command

route add [-net|-host] 目的主机或目的网络 \
> [netmask 子网掩码] \
> [gw 下一跳地址]  \
> [[dev] 接口地址]

to demonstrate adding a specific host. Routing

# route add -host 192.168.110.111  gw 192.168.110.1 dev eth0

In this way, a piece of routing information is added successfully.

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