Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

The whole site in one document for easy reference!
The TCP/IP Guide

Custom Search







Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)
                     9  IP Datagram Delivery and Routing

Previous Topic/Section
IP Routing Concepts and the Process of Next-Hop Routing
Previous Page
Pages in Current Topic/Section
1
23
Next Page
IP Routing In A Subnet Or Classless Addressing (CIDR) Environment
Next Topic/Section

IP Routes and Routing Tables
(Page 1 of 3)

Routers are responsible for forwarding traffic on an IP internetwork. Each router accepts datagrams from a variety of sources, examines the IP address of the destination and decides what the next hop is that the datagram needs to take to get it that much closer to its final destination. A question then naturally arises: how does a router know where to send different datagrams?

Each router maintains a set of information that provides a mapping between different network IDs and the other routers to which it is connected. This information is contained in a data structure normally called a routing table. Each entry in the table, unsurprisingly called a routing entry, provides information about one network (or subnetwork, or host). It basically says “if the destination of this datagram is in the following network, the next hop you should take is to the following device”. Each time a datagram is received the router checks its destination IP address against the routing entries in its table to decide where to send the datagram, and then sends it on its next hop.

Obviously, the fewer the entries in this table, the faster the router can decide what to do with datagrams. (This was a big part of the motivation for classless addressing, which aggregates routes into “supernets” to reduce router table size, as we will see in the next topic.) Some routers only have connections to two other devices, so they don't have much of a decision to make. Typically, the router will simply take datagrams coming from one of its interfaces and if necessary, send them out on the other one. For example, consider a small company's router acting as the interface between a network of three hosts and the Internet. Any datagrams sent to the router from a host on this network will need to go over the router's connection to the router at the ISP.

When a router has connections to more than two devices, things become considerably more complex. Some distant networks may be more easily reachable if datagrams are sent using one of the routers than the other. The routing table contains information not only about the networks directly connected to the router, but also information that the router has “learned” about more distant networks.

Key Concept: A router make decisions about how to route datagrams using its internal routing table. The table contains entries specifying to which router datagrams should be sent to reach a particular network.



Previous Topic/Section
IP Routing Concepts and the Process of Next-Hop Routing
Previous Page
Pages in Current Topic/Section
1
23
Next Page
IP Routing In A Subnet Or Classless Addressing (CIDR) Environment
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.