Understanding the network.pdf - Back to Home

1. The Layer 3 to Layer 2 address mapping lookup is complete.
2. The IP datagram is placed in the Layer 2 packet.
3. The datagram is forwarded directly to the destination host.
If the datagram's destination is remotely connected:
NOTE
1. The Layer 3 to Layer 2 address mapping lookup is complete.
2. The IP datagram is placed in the Layer 2 packet.
3. The datagram is sent directly to default gateway.
Every time an IP datagram is forwarded, the host/router must perform a route
lookup to determine the next hop. This is done so the best route will be used to
deliver the IP datagram. Although this might seem like a waste of time on an
Ethernet network where each subnet is connected by only one Ethernet interface, it
is not a waste of time where there is a choice between sending the datagram over
a 56K ISDN dial-up link or a TI dedicated link. Because IP has no knowledge of the
Layer 2 transport, it handles all datagrams the same way.
It is important to keep in mind that IP datagram delivery is a Layer 2 and Layer 3
activity. The IP protocol is uninterested in the actual physical layer transport. This
gives IP the flexibility to send datagrams across any physical transport medium,
provided there is a means to provide physical transport (OSI Layer 2) to network
(OSI Layer 3) address mapping. The address resolution protocol is the most
common example of this service and is used with most Layer 2 LAN protocols
(Ethernet, FDDI, and so on). ARP, as you recall from the earlier discussion, provides
the mapping service used to map the Layer 3 addresses to Layer 2 addresses.
Mapping is needed because the Layer 2 address is only valid for the physical
transport segment to which the source host (and, in the case of local delivery, the
destination host) is attached. In other words, the Layer 2 address associated with
an IP datagram changes each time the IP datagram is moved between intermediate
points. The Layer 3 addresses, however, are always valid. They never change,
except in the case where a proxy host or NAT service is used.
The dependency on both Layer 2 and Layer 3 addressing for IP datagram delivery
can sometimes be confusing. It's important to be clear about the IP delivery models
and the roles that the Layer 2 and Layer 3 addressing play.
• In instances where local delivery is used, the source and destination
hosts'Layer 2 and Layer 3 addresses correspond to one another because the
delivery is local.