Chapter 6 – Routing

What is IP Routing? IP Routing is the process of sending data from a host on one network to a remote host on another network through a router, or routers Routers are referred to as gateways Gateways have two or more network connections

Routing Table Each router maintains a routing table The routing table specifies the path that the router uses to deliver a packet to a specified remote host Routing tables maintain entries of where networks are located, not hosts ARP is used in determining routing table entries

Dynamic versus Static Routing There are two types of routing tables: static tables and dynamic tables. Static tables are maintained by network manager Dynamic tables are maintained by a routing protocol

Windows NT Server 4.0 as a router NT Server can perform as an IP router using either static or dynamic routing NT server can be configured with multiple network adapters and can route between them This arracngement is referred to as a multihoned computer Supports dynamic routing through use of RIP (Routing Information Protocol)

Static Routing Routing tables are built and maintained manually If a route changes, static routers do not share this information with each other Static routing tables consist of five columns of data

Five Columns in Routing Table Destination- Can be an IP network or host address Network mask – Subnet mask associated with Destination IP address Forwarding IP Address – IP address to which the datagram is to be forwarded Interface – Logical or physical interface used when forwarding datagram. Windows 2000 uses uses the IP address of the interface Metric – Estimate of the number of hops (the number of routers to the destination)

Route Determination Process Determine which routes in the routing table match the destination IP address by performing logical AND operation on destination IP address and Network Mask (in 2nd column of table). Compare results with Destination filed From those routes that match, select the route(s) having the most number of 1 bits in the Network Mask field From the step above, select route with the lowest metric

Route Determination Process – Cont. The above process results in a single route The single route chosen yield the forwarding IP address and interface over which to forward the datagram If no match is found, IP indicates a routing error You can see the routing table using the command: route print

Some Specific Routes In Routing Table Destination and Network mask of 0.0.0.0 is the default when there are no other matches. The packet is in this case forwarded to the default gateway Destination of 127.0.0.0 and Network Mask of 255.0.0.0 is the Loopback Network Route where all datagrams are forwarded to the reserved loopback address 127.0.0.1

Specific Routes – Cont. Route to local subnet is indicated by Destination having local host IP address and local host subnet. Local Host route has IP address of local host for Destination and 255.255.255.255 for subnet mask

Windows 2000 IP Routing Process Perform the route determination process described above to choose a single route From the single route, examine the gateway and interface IP addresses If these are the same, set the forwarding IP address to the original destination IP address If they are different, set the forwarding IP address to the gateway IP address The forwarding IP address is then sent to ARP to determine the MAC address associated with this IP address

Example of Routing Table Use route print command to vies routing table of your machine

Examples of Windows 2000 Route Determination See Page 150 of text for sample routing table Example of Local traffic Example of Remote traffic