Understanding the network.pdf - Back to Home

IP also supports a facility for hosts to masquerade and accept IP packets on behalf
of other hosts. This is known as Proxy ARP. Proxy ARP is used in situations where
sending hosts are unable to determine that IP routing is required. Proxy ARP maps
one or more network prefixes to a physical Ethernet address. This permits the host
running Proxy ARP to answer all ARP requests made for the IP network for which it
acts as proxy. Proxy ARP is a limited type of routing and is rarely found in modern
networks. Its original intent was to assist in situations where host IP
implementations were limited and to provide the ARP facility for
non-ARP-compatible transport links.
With local datagram delivery, the Layer 2 protocol handles most of the work. IP and
its supporting protocols act as address translation mechanisms that assist Layer 2 in
identifying the source and destination hosts for the delivery of the ULP data. The
main function of IP (and all other Layer 3 protocols) is to provide a common
transmission interface between the ULP and the Layer 2 transmission medium. This
function is essential in order for hosts to have the capability to exchange data
independent of the actual transmission protocol. The utilization of this common
interface makes it possible for the exchange of data to occur over different
transmission mediums utilizing different addressing, data encoding, and
transmission schemes. In a LAN environment, this is often not a large concern
because one transmission medium is used to connect the different hosts on the
network. However, when data needs to be delivered to a host beyond the local
network, a remote delivery model comes into play. Remote datagram delivery
involves the possible use of different Layer 2 transmission protocols and the need
for a mechanism for determining how to reach the remote host, known as routing.
IP Header
Now that you have an understanding of IP datagram delivery and its interaction with
the Layer 2 protocol, let's look at how the IP header is structured. An IP header is
appended to each actual upper layer protocol message that needs to be delivered
across the network. In most cases, the IP header and its various parts have little
effect on your environment beyond delivering your IP data. But, understanding the
parts of the IP header gives you insight into how IP works in general. Figure 2.2
illustrates the different data fields that make up the IP header, followed by a brief
description of each field's function.