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This chapter is from the book

1.2 Internetworking Basics

To facilitate the reader's understanding, this section covers some of the notation and conventions used in the book and thus indicates the level of the typical reader's internetworking knowledge anticipated by the authors.

Throughout the book we use the slash notation for bit mask when describing IP address ranges. The slash notation indicates how many bits of the address remain constant throughout the range of addresses. For example, 10.0.0.0/8 indicates a range of IP addresses all with the first 8 bits equal to 10. The range is from address 10.0.0.0 to 10.255.255.255.

We also make reference to classful networks. The class A, B, and C networks constitute all unicast IP addresses as follows:

  • Class A networks: Describe the range of networks from 1.0.0.0/8 through 126.0.0.0/8.

  • Class B networks: Describe the range of networks from 128.0.0.0/16 through 191.255.0.0/16.

  • Class C networks: Describe the range of networks from 192.0.0.0/24 through 223.255.255.0/24.

Originally, networks were assigned to organizations along classful boundaries. That meant class A networks were assigned in /8 blocks, class B in /16 blocks, and class C in /24 blocks. Classful allocation was inefficient because organizations that required slightly more than 254 addresses could be assigned an entire class B. Classless interdomain routing (CIDR) enabled the assignment and routing of addresses outside of classful boundaries. An organization that needed enough addresses for 500 hosts could be assigned one /23, instead of an entire class B network.

All multicast addresses fall in the class D range of the IPv4 address space. The class D range is 224.0.0.0 through 239.255.255.255. Multicast addresses do not have a mask length associated with them for forwarding purposes. Each address is treated independently so the mask used for forwarding is always assumed to be /32. We use shorter mask lengths on multicast addresses in some parts of the book for reasons other than forwarding. These masks generally are used to describe ranges of multicast addresses. For example, the address range reserved for Source-Specific Multicast (SSM) is 232.0.0.0/8.

We refer throughout the book to unicast and multicast routing protocols. Unicast routing protocols are used by routers to exchange routing information and build routing tables. Unicast IP routing protocols are further categorized into interior gateway protocols (IGPs) and exterior gateway protocols (EGPs).

IGPs provide routing within an administrative domain known as an autonomous system (AS). EGPs provide routing between ASs. Routing Information Protocol (RIP), Open Shortest Path First (OSPF), and Intermediate System to Intermediate System (IS-IS) are examples of IGPs, while Border Gateway Protocol (BGP) is an example of an EGP. Multicast routing protocols are used by routers to set up multicast forwarding state and to exchange this information with other multicast routers. Examples of multicast IP routing protocols are Distance Vector Multicast Routing Protocol (DVMRP), Protocol Independent Multicast–Dense Mode (PIM-DM), and Protocol Independent Multicast–Sparse Mode (PIM-SM).

The terms control packets and data packets are used to differentiate the types of packets being routed through the network. Control packets include any packets sent for the purpose of exchanging information between routers about how to deliver data packets through the network. Control packets are typically protocol traffic that network devices use to communicate with one another to make such things as routing possible.

Data packets use the network to communicate data between hosts; they do not influence the way the network forwards traffic. Letters delivered via postal mail are analogous to data packets. Information exchanged between post offices to describe what ZIP codes mean is analogous to control packets. In the IP world, all packets sent for an FTP session between hosts are considered data packets, while a BGP Update message is an example of a control packet.

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