1. Chapter 6 Delivery & Forwarding of IP Packets
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2. ROUTING METHODS There are various routing methods:
Next-Hop Routing – table only holds the address of the next hop (instead info regarding the entire route) – routing table for each host
Network-Specific Routing – instead of an entry for each host (on the same network), only one entry for the network is defined
Host-Specific Routing – for a specific destination host, you might want to control the exact route – in this case, the actual Rx is listed in the routing table and the desired next hop is listed
Default Routing – instead of listing all of the various networks in the Internet, Tx host would use one entry called the Default (network address )
Dr. Clincy

3. Next-hop routing
Next-Hop Routing – table only holds the address of the next hop (instead info regarding the entire route)
Show more routers in better illustrating the routing table
Dr. Clincy

4. Network-specific routing
Network-Specific Routing – instead of an entry for each host on the same physical network, only one entry for the network is defined
Dr. Clincy

5. Host-specific Routing
Host-Specific Routing – for a specific destination host, you might want to control the exact route – in this case, the actual Rx is listed in the routing table and the desired next hop is listed R1
In this case, you want every packet traveling to Host B to traverse through R3. For the other hosts on N2 and N3, the Network-specific routing approach is used.
Dr. Clincy

6. Default Routing
Default Routing – instead of listing all of the various networks in the Internet, Tx host would use one entry called the Default (network address )
In this case, R1 sends to a specific network however, R2 sends to the remainder of the Internet (default)
Dr. Clincy

7. Simplified forwarding module in classful address without subnetting
Using the next-hop address and interface number, ARP searches for the physical address in facilitating the actual hop
For the Classful case, per router, a table was needed for each class – this made the searching simple
Dr. Clincy

8. Configuration for routing for R1, Classful Case
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9. Simplified forwarding module in classful address with subnetting
Recall for the Classful case, subnetting is done within the organization
Dr. Clincy

10. Configuration for the Classful and Subnetting Case
Doesn’t know what network is connected to router here
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11. Simplified forwarding module in classless address
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12. Routing Table for R1 in the Illustrated Configuration – Classless Case
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13. Address aggregation
With the classless approach, routing tables increased – in reducing the size of some tables, use a router to represent multiple blocks – address aggregation
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14. STATIC VERSUS DYNAMIC ROUTING
Host or router uses a routing table
Table can be either static or dynamic in nature
A static routing table contains information entered manually.
A dynamic routing table is updated periodically using one of the dynamic routing protocols such as RIP, OSPF, or BGP
Regarding dynamic routing table: if fiber cut or router failure, the tables are updated
Dr. Clincy

15. Router’s Table Logistics
When the router is looking for the route, it:
First check for direct delivery
Then host-specific delivery,
The network-specific delivery, and
Finally, default delivery
This order can be organized with in the routing table
Dr. Clincy

16. Routing Table
Mask: used to extract the net id of the Rx. For Host-Specific Routing - the mask is and for Default Routing – the mask is
Destination Address: either the destination host address or destination network address
Next-hop Address: next hop router address
Flags
U - The router is up and running. If router is down, the packet discarded
G - The destination is in another network. If G flag present, indirect delivery (if not, direct delivery)
H – If H flag present, destination field contains Host-specific address (if not present, network address)
D – If D flag present, routing info added to host routing table via ICMP’s redirection (cover later)
M - If M flag present, routing info was modified via ICMP’s redirection (cover later)
Reference count: # of users using this route at any moment
Use: # of packets transmitted through this router for the corresponding Rx
Interface: name of the interface
Dr. Clincy