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 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

IP Routes and Routing Tables 1 2 IP Multicasting

Aggregated Routes and their Impact on Routing

Classless addressing is formally called Classless Inter-Domain Routing or CIDR. The name mentions routing and not addressing, and this is evidence that CIDR was introduced in large part to improve the efficiency of routing. This improvement occurs because classless networks use a multiple-level hierarchy. Each network can be broken down into subnetworks, sub-subnetworks, and so on. This means that when we are deciding how to route in a CIDR environment, we can also describe routes in a hierarchical manner. Many smaller networks can be described using a single, higher-level network description that represents them all to routers in the rest of the internet. This technique, sometimes called route aggregation, reduces routing table size.

Let's refer back to the detailed example I gave in the addressing section on CIDR. An ISP started with the block 71.94.0.0/15 and subdivided it multiple times to create smaller blocks for itself and its customers. To the customers and users of this block, these smaller blocks must be differentiated; the ISP obviously needs to know how to route traffic to the correct customer. To everyone else on the Internet, however, these details are unimportant in deciding how to route datagrams to anyone in that ISP's block. For example, suppose I am using a host with IP address 211.42.113.5 and I need to send to 71.94.1.43. My local router, and the main routers on the Internet, don't know where in the 71.94.0.0/15 block that address is, and they don't need to know either. They just know that anything with the first 15 bits containing the binary equivalent of 71.94 goes to the router(s) that handle(s) 71.94.0.0/15, which is the aggregated address of the entire block. They let the ISP's routers figure out which of its constituent subnetworks contains 71.94.1.43.

Contrast this to the way it would be in a “classful” environment. Each of the customers of this ISP would probably have one or more Class C address blocks. Each of these would require a separate router entry, and these blocks would have to be known by all routers on the Internet. Thus, instead of just one 71.94.0.0/15 entry, there would be dozens or even hundreds of entries for each customer network. In the classless scheme, only one entry exists, for the “parent” ISP.

CIDR provides benefits to routing but also increases complexity. Under CIDR, we cannot determine which bits are the network ID and which the host ID just from the IP address. To make matters worse, we can have networks, subnetworks, sub-subnetworks and so on that all have the same base address!

In our example above, 71.94.0.0/15 is the complete network, and subnetwork #0 is 71.94.0.0/16. They have a different prefix length (the number of network ID bits) but the same base address. If a router has more than one match for a network ID in this manner, it must use the match with the longest network identifier first, since it represents a more specific network description.

IP Routes and Routing Tables 1 2 IP Multicasting