This module deals with TCP/IP routing and how network administrators use subnets to segment their networks to give greater flexibility in their networks. The concept of subnetting, or the borrowing of bits from the host field, to create additional networks is covered in depth.

Key functions of a router:

• Routers must maintain routing tables and make sure other routers know of changes in the network topology. They use routing protocols to communicate network information with other routers.
• When packets arrive at an interface, the router must use the routing table to determine where to send them. The router switches the packets to the appropriate interface, adds the frame information for the interface, and then transmits the frame

Contents of an IP Packet:

• Version
• IP header length (HLEN)
• Type of service
• Total length
• Identification
• Flags
• Fragment offset
• Time to Live (TTL)
• Protocol
• Header checksum
• Source address
• Destination address
• Options
• Padding
• Data

• Protocol-A set of rules, or guidelines, followed by a router. Defined by Cisco as the format of a message or the way that computers exchange messages for specific activities.
• Routed Protocol-A protocol that can be routed by a router that is used to direct user traffic.
• Routing Protocol-A protocol that allows communication between routers to share information such as routes, updates to maintain routing tables.
  
• Routing metric – Different routing protocols use different routing metrics. Routing metrics are used to determine the desirability of a route. For example, RIP uses hop count as its only routing metric. IGRP uses bandwidth, load, delay, and reliability metrics to create a composite metric value.
• Path Determination-Routers use path determination to compare a destination address to the available routes in its routing table and to select the best path to send data.
• Hop Count-The number of routers a packet travels through. One hop count equates to a packet being sent through one router.
  

Key Concepts:

Routing Protocols

There are two types of routing protocols: Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs). IGPs are used within an autonomous network (a network under common administrative control) and EGPs are used between IGPs to connect them. IGPs can be defined as either distance-vector or link-state protocols.