Sybex - CCNA 2.0 Study Guide (640-507).pdf - Cifo

TCP/IP and the DoD Model 117
layer. The Internet layer’s second job is to provide a single network interface
to the upper-layer protocols. Without this layer, application programmers
would need to write “hooks” into every one of their applications for each different
Network Access protocol. This would not only be a pain in the neck, but
it would lead to different versions of each application—one for Ethernet,
another one for Token Ring, and so on. To prevent this, IP provides one single
network interface for the upper-layer protocols. That accomplished, it’s then
the job of IP and the various Network Access protocols to get along and work
together.
All network roads don’t lead to Rome—they lead to IP. And all the other
protocols at this layer, as well as all those at the upper layers, use it. Never
forget that. All paths through the model go through IP. The following sections
describe the protocols at the Internet layer.
These are the protocols that work at the Internet layer:
Internet Protocol (IP)
Internet Control Message Protocol (ICMP)
Address Resolution Protocol (ARP)
Reverse Address Resolution Protocol (RARP)
Internet Protocol (IP)
The Internet Protocol (IP) essentially is the Internet layer. The other protocols
found here merely exist to support it. IP contains the big picture and
could be said to “see all,” in that it is aware of all the interconnected networks.
It can do this because all the machines on the network have a software,
or logical, address called an IP address, which we’ll cover more
thoroughly later in this chapter.
IP looks at each packet’s address. Then, using a routing table, it decides
where a packet is to be sent next, choosing the best path. The Network
Access–layer protocols at the bottom of the model don’t possess IP’s enlightened
scope of the entire network; they deal only with physical links (local
networks).
Identifying devices on networks requires answering these two questions:
Which network is it on? And what is its ID on that network? The first answer
is the software, or logical, address (the correct street). The second answer is
the hardware address (the correct mailbox). All hosts on a network have a
logical ID called an IP address. This is the software, or logical, address and