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Chapter 3
Internet Protocols
to "see all," in that it's 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 I'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 protocols of the
Network Access layer at the bottom of the DoD 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 address, 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 contains valuable encoded information greatly simplifying the complex
task of routing. (Please note that IP is discussed in RFC 791.)
IP receives segments from the Host-to-Host layer and fragments them into
datagrams (packets). IP then reassembles datagrams back into segments on
the receiving side. Each datagram is assigned the IP address of the sender and
of the recipient. Each router (layer-3 device) that receives a datagram makes
routing decisions based on the packet's destination IP address.
Figure 3.6 shows an IP header. This will give you an idea of what the IP
protocol has to go through every time user data is sent from the upper layers
and is to be sent to a remote network.
F I G U R E 3 . 6
IP header
Bit 0
Bit 15
Total length (16)
Header checksum (16)
Time to Live (8)
Protocol (8)
Version
(4)
Flags
(3)
Header
length (4)
Priority and
Type of Service (8)
Identification (16)
Fragment offset (13)
Options (0 or 32 if any)
Destination IP address (32)
Source IP address (32)
Data (varies if any)
Bit 16
Bit 31
20 bytes
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