Summary of Module 10

Summary
This page summarizes the topics discussed in this module.


IP is referred to as a connectionless protocol because no dedicated circuit connection is established between source and destination prior to transmission, IP is referred to as unreliable because does not verify that the data reached its destination. If verification of delivery is required then a combination of IP and a connection-oriented transport protocol such as TCP is required. If verification of error-free delivery is not required IP can be used in combination with a connectionless transport protocol such as UDP. Connectionless network processes are often referred to as packet switched processes. Connection-oriented network processes are often referred to as circuit switched processes.

Protocols at each layer of the OSI model add control information to the data as it moves through the network. Because this information is added at the beginning and end of the data, this process is referred to as encapsulating the data. Layer 3 adds network, or logical, address information to the data and Layer 2 adds local, or physical, address information.

Layer 3 routing and Layer 2 switching are used to direct and deliver data throughout the network. Initially, the router receives a Layer 2 frame with a Layer 3 packet encapsulated within it. The router must strip off the Layer 2 frame and examine the Layer 3 packet. If the packet is destined for local delivery the router must encapsulate it in a new frame with the correct local MAC address as the destination. If the data must be forwarded to another broadcast domain, the router must encapsulate the Layer 3 packet in a new Layer 2 frame that contains the MAC address of the next internetworking device. In this way a frame is transmitted through networks from broadcast domain to broadcast domain and eventually delivered to the correct host.

Routed protocols, such as IP, transport data across a network. Routing protocols allow routers to choose the best path for data from source to destination. These routes can be either static routes, which are entered manually, or dynamic routes, which are learned through routing protocols. When dynamic routing protocols are used, routers use routing update messages to communicate with one another and maintain their routing tables. Routing algorithms use metrics to process routing updates and populate the routing table with the best routes. Convergence describes the speed at which all routers agree on a change in the network.

Interior gateway protocols (IGP) are routing protocols that route data within autonomous systems, while exterior gateway protocols (EGP) route data between autonomous systems. IGPs can be further categorized as either distance-vector or link-state protocols. Routers using distance-vector routing protocols periodically send routing updates consisting of all or part of their routing tables. Routers using link-state routing protocols use link-state advertisements (LSAs) to send updates only when topological changes occur in the network, and send complete routing tables much less frequently.

As a packet travels through the network devices need a method of determining what portion of the IP address identifies the network and what portion identifies the host. A 32-bit address mask, called a subnet mask, is used to indicate the bits of an IP address that are being used for the network address. The default subnet mask for a Class A address is 255.0.0.0. For a Class B address, the subnet mask always starts out as 255.255.0.0, and a Class C subnet mask begins as 255.255.255.0. The subnet mask can be used to split up an existing network into subnetworks, or subnets.

Subnetting reduces the size of broadcast domains, allows LAN segments in different geographical locations to communicate through routers and provides improved security by separating one LAN segment from another.

Custom subnet masks use more bits than the default subnet masks by borrowing these bits from the host portion of the IP address. This creates a three-part address:

• The original network address

• The subnet address made up of the bits borrowed

• The host address made up of the bits left after borrowing some for subnets

Routers use subnet masks to determine the subnetwork portion of an address for an incoming packet. This process is referred to as logical ANDing.