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What is a routing table?

Posted On 2008-02-03 by FortyPoundHead
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Tags: Networking Tutorial 
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A routing table is used by TCP/IP network routers to calculate the destinations of messages it is responsible for forwarding. The table is a small in-memory database managed by the router''s built-in hardware and software.

Routing Table Entries and Sizes
Routing tables contain a list of IP addresses. Each IP address identifies a remote router (or other network gateway) that the local router is configured to recognize. For each IP address, the routing 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 routing 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 contain ten or fewer entries. By comparison, the largest routers at the core of the Internet backbone must maintain the full Internet routing table that exceeds 100,000 entries and growing as the Internet expands.

Two hypothetical, partial routing table entries are shown below:
IP Address: 172.48.11.181 - Network Mask: 255.255.255.255
IP Address: 192.168.1.1 - Network Mask: 255.255.255.0

In this example, the first entry represents the route to the ISP''s primary DNS server. Requests made from the home network to any destination on the Internet will be sent to the IP address 172.48.11.181 for forwarding. The second entry represents the route between any computers within the home network, where the home router has IP address 192.168.1.1.

Dynamic vs. Static Routing
Home routers set up their routing tables automatically when connected to the ISP, a process called dynamic routing. They generate one routing table entry for each of the ISPs 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.

Most residential network routers prevent you from manually overriding or changing the routing table. However, business routers typically allow network administrators to manually update or manipulate routing tables. This so-called static routing can be useful when optimizing for network performance and reliability.

Viewing the Contents of Routing Tables
On home broadband routers, the routing table contents are typically shown on a screen inside the administrative console.

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


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