What are TCP/IP Router (Routing) Tables?

Advanced Routing - Linksys WRT54GS
Advanced Routing - Linksys WRT54GS.

A router table (also called routing table) is stored data used by TCP/IP network routers to calculate the destinations of messages they are responsible for forwarding. A router table is a small in-memory database managed by the router's built-in hardware and software.

Router Table Entries and Sizes

Router tables contain a list of IP addresses. Each address in the list identifies a remote router (or other network gateway) that the local router is configured to recognize.

For each IP address, the router table additionally stores a network mask and other data that specifies the destination IP address ranges that remote device will accept.

Home network routers utilize a very small router table because they simply forward all outbound traffic to the Internet Service Provider (ISP) gateway which takes care of all other routing steps. Home router tables typically contains ten or fewer entries. By comparison, the largest routers at the core of the Internet backbone must maintain the full Internet routing table that contains several hundred thousand entries. (See the CIDR Report for latest Internet routing statistics.)

Dynamic vs. Static Routing

Home routers set up their routing tables automatically when connected to the Internet provider, a process called dynamic routing. They generate one router table entry for each of the service provider's DNS servers (primary, secondary and tertiary if available) and one entry for routing among all the home computers.

They may also generate a few additional routes for other special cases including multicast and broadcast routes.

Some residential network routers prevent you from manually overriding or changing the router table. However, business routers allow network administrators to manually updates or manipulate routing tables.

This so-called static routing can be useful when optimizing for network performance and reliability. On a home network, the use of static routes are not required except in unusual circumstances (such as when setting up multiple subnetworks and a second router).

Viewing the Contents of Routing Tables

On home broadband routers, routing table contents are typically shown on a screen inside the administrative console. An example IPv4 table is shown below.

Routing Table Entry List (Example)
Destination LAN IPSubnet MaskGatewayInterface (Internet)
xx.yyy.86.1255.255.255.255xx.yyy.86.1WAN (Internet)
xx.yyy.86.134255.255.255.255xx.yy.86.134WAN (Internet) & Wireless

In this example, the first two entries represent routes to the Internet provider's gateway address ('xx' and 'yyy' represent real IP address values that are hidden for the purpose of this article). The third entry represents the route to the home router's public facing IP address assigned by the provider. The last entry represents the route for all computers within the home network to the home router, where the router has IP address

On Windows and Unix/Linux computers, the netstat -r command also displays the contents of the router table configured on the local computer.

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