The Purpose and Inner Workings of TCP/IP Router Tables GA S REGULAR Menu Lifewire Tech for Humans Newsletter! Search Close GO Internet, Networking, & Security > Home Networking 28 28 people found this article helpful

What Are TCP/IP Router (Routing) Tables?

Router tables determine the destination of messages within a network

By Bradley Mitchell Bradley Mitchell Writer Massachusetts Institute of Technology University of Illinois An MIT graduate who brings years of technical experience to articles on SEO, computers, and wireless networking. lifewire's editorial guidelines Updated on February 28, 2020 Tweet Share Email Tweet Share Email Home Networking The Wireless Connection Routers & Firewalls Network Hubs ISP Broadband Ethernet Installing & Upgrading Wi-Fi & Wireless A router table (also called routing table) helps TCP/IP network routers to calculate the destination of the 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 another 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 devices will accept. Home network routers employ a very small router table because they simply forward all outbound traffic to the Internet Service Provider 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 contains several hundred thousand entries. See the CIDR Report for the latest internet routing statistics.

Dynamic vs Static Routing

Home routers set up their routing tables automatically when they connect 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 update 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 routers is not required except in unusual circumstances (such as when you establish several 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. Destination LAN IP Subnet Mask Gateway Interface 0.0.0.0 0.0.0.0 xx.yyy.86.1 WAN (Internet) xx.yyy.86.1 255.255.255.255 xx.yyy.86.1 WAN (Internet) xx.yyy.86.134 255.255.255.255 xx.yyy.86.134 WAN (Internet) 192.168.1.0 255.255.255.0 192.168.1.101 LAN & Wireless Routing Table Entry List (Example) 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 example). 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 192.168.1.101. On Windows and Unix/Linux computers, the netstat -r command displays the contents of the router table configured on the local computer. Was this page helpful? Thanks for letting us know! Get the Latest Tech News Delivered Every Day Subscribe Tell us why! Other Not enough details Hard to understand Submit More from Lifewire What Is an IP Address? How to Find the Default IP Address of a Belkin Router What Is a Public IP Address? (and How to Find Yours) One Home Network Sharing Two Internet Connections How to Set Up a Router How to Use an IP Address to Find a MAC Address How to Connect Two Routers on a Home Network Can Two Routers Be Used on the Same Home Network? How to Set Up PPPoE Internet Access How to Change Your IP Address Private IP Addresses: Everything You Need to Know What Is a Router and How Does It Work? How to Find Your Default Gateway IP Address How to Set Up Port Forwarding How to Obtain a Fixed IP Address 127.0.0.1 IP Address Explained Newsletter Sign Up Newsletter Sign Up Newsletter Sign Up Newsletter Sign Up Newsletter Sign Up By clicking “Accept All Cookies”, you agree to the storing of cookies on your device to enhance site navigation, analyze site usage, and assist in our marketing efforts. Cookies Settings Accept All Cookies