Routable and routed protocols
10.1.1 This page will define routed and routable protocols.
A protocol is a set of rules that determines how computers communicate with each other across networks. Computers exchange data messages to communicate with each other. To accept and act on these messages, computers must have sets of rules that determine how a message is interpreted. Examples include messages used to establish a connection to a remote machine, e-mail messages, and files transferred over a network.
A protocol describes the following:
• The required format of a message
• The way that computers must exchange messages for specific activities
A routed protocol allows the router to forward data between nodes on different networks. A routable protocol must provide the ability to assign a network number and a host number to each device. Some protocols, such as IPX, require only a network number. These protocols use the MAC address of the host for the host number. Other protocols, such as IP, require an address with a network portion and a host portion. These protocols also require a network mask to differentiate the two numbers. The network address is obtained by ANDing the address with the network mask.
The reason that a network mask is used is to allow groups of sequential IP addresses to be treated as a single unit. If this grouping were not allowed, each host would have to be mapped individually for routing. This would be impossible, because according to the Internet Software Consortium there are approximately 233,101,500 hosts on the Internet.
The next page will discuss IP.
10.1.2 This page describes the features and functions of IP.
IP is the most widely used implementation of a hierarchical network-addressing scheme. IP is a connectionless, unreliable, best-effort delivery protocol. The term connectionless means that no dedicated circuit connection is established prior to transmission. IP determines the most efficient route for data based on the routing protocol. The terms unreliable and best-effort do not imply that the system is unreliable and does not work well. They indicate that IP does not verify that data sent on the network reaches its destination. If required, verification is handled by upper layer protocols.
As information flows down the layers of the OSI model, the data is processed at each layer. At the network layer, the data is encapsulated into packets. These packets are also known as datagrams. IP determines the contents of the IP packet header, which includes address information. However, it is not concerned with the actual data. IP accepts whatever data is passed down to it from the upper layers.
The next page examines how a packet travels through a network
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