Understanding the network.pdf - Back to Home

• Under the remote delivery model, the source and destination Layer 2
addresses changes each time the IP datagram is forwarded through an
intermediate router. The Layer 3 source always reflects the originating host,
and the Layer 3 destination address is always that of the actual destination
host.
Before moving on, here is a review of the IP datagram delivery. In Figure 2.10, a
private WAN uses a VLSM subnetted Class B 172.168.0.0 network and two natural
Class C networks: 192.168.20.0 and 192.168.21.0. There is also a host on the
172.168.0.0 /24 subnet that sends a datagram to a host on the 192.168.21.0 /24
subnet.
Figure 2.10. Another example network.
Use this example to review the various processes needed to deliver the datagram:
1. Starting at the sending host—The host performs a route lookup. No match is
found, so the network is not local. The host then performs an ARP lookup to
find the gateway's Layer 2 address. The IP datagram (source: 172.168.0.14;
destination: 192.168.21.23) is placed into an Ethernet frame (source: a3;
destination: 25f) and forwarded to the default gateway. This begins the
remote delivery process.
2. Router A—Router A receives the packet on interface e0. It strips out the IP
datagram, discards the Ethernet frame and performs a route table lookup.
The next hop is 172.168.1.1 /24 via interface e1. The datagram is placed
into a new Ethernet frame (source: 67f; destination: a1) and forwarded to
the next hop. Because all datagram delivery is based on the "next hop," the
router does not know or care if the actual destination is the next hop or 10
hops away. It is only concerned with building a table that (to its knowledge)
lists forwarding points to the networks that are part of its internetwork. After
the datagram is forwarded, it is the next hop's responsibility to deliver the
datagram.