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!

Get The TCP/IP Guide for your own computer.
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 Datagram Direct Delivery and Indirect Delivery (Routing)
Previous Page
Pages in Current Topic/Section
1
2
Next Page
IP Routes and Routing Tables
Next Topic/Section

IP Routing Concepts and the Process of Next-Hop Routing
(Page 2 of 2)

The Benefits of Next-Hop Routing

This is a critical concept in how IP works: routing is done on a step-by-step basis, one hop at a time. When we decide to send a datagram to a device on a distant network, we don't know the exact path that the datagram will take; we only have enough information to send it to the correct router to which we are attached. That router, in turn, looks at the IP address of the destination and decides where the datagram should next “hop” to. This process continues until the datagram reaches the destination host's network, when it is delivered.

Next-hop routing may seem at first like a strange way of communicating datagrams over an internetwork. In fact, it is part of what makes IP so powerful. On each step of the journey to any other host, a router only needs to know where the next step for the datagram is. Without this concept, each device and router would need to know what path to take to every other host on the internet, which would be quite impractical.

Key Concept: Indirect delivery of IP datagrams is accomplished using a process called next-hop routing, where each message is handed from one router to the next until it reaches the network of the destination. The main advantage of this is that each router needs only to know which neighboring router should be the next recipient of a given datagram, rather than needing to know the exact route to every destination network.


Datagram Processing At Each Hop

As mentioned above, each “hop” in routing consists of traversal of a physical network. After a source sends a datagram to its local router, the data link layer on the router passes it up to the router's IP layer. There, the datagram's header is examined, and the router decides what the next device is to send the datagram to. It then passes it back down to the data link layer to be sent over one of the router's physical network links, typically to another router. The router will either have a record of the physical addresses of the routers to which it is connected, or it will use ARP to determine these addresses.

The Essential Role of Routers in IP Datagram Delivery

Another key concept related to the principle of next-hop routing is that routers are designed to accomplish routing, not hosts. Most hosts are connected using only one router to the rest of the internet (or Internet). It would be a maintenance nightmare to have to give each host the smarts to know how to route to every other host. Instead, hosts only decide if they are sending locally to their own network, or if they are sending to a non-local network. If the latter, they just send the datagram to their router and say “here, you take care of this”. If a host has a connection to more than one router, it only needs to know which router to use for certain sets of distant networks. How routers decide what to do with the datagrams when they receive them from hosts is the subject of the next topic.


Previous Topic/Section
IP Datagram Direct Delivery and Indirect Delivery (Routing)
Previous Page
Pages in Current Topic/Section
1
2
Next Page
IP Routes and Routing Tables
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.