CCNA Cisco Certified Network Associate Study Guide - FTP Server

118 Chapter 3 � Internet Protocol
FIGURE 3.6 IP header
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 upon 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 wants to be sent to a remote network.
Bit 0 Bit 15
Version
(4)
Header
length (4)
Priority and
Type of Service (8)
Time to Live (8) Protocol (8)
The following fields make up the IP header:
Version IP version number.
HLEN Header length in 32-bit words.
Priority or ToS Type of Service tells how the datagram should be handled.
The first three bits are the priority bits.
Total length Length of the packet including header and data.
Identification Unique IP-packet value.
Total length (16)
Flags
Identification (16) Fragment offset (13)
(3)
Source IP address (32)
Destination IP address (32)
Options (0 or 32 if any)
Data (varies if any)
Bit 16 Bit 31
Header checksum (16)
20 bytes