Information Technology Reference
In-Depth Information
This chapter reviews IPv4 headers, address classes, and assignment methods.
IP is the network layer protocol in TCP/IP. It contains logical addressing and information
for routing packets throughout the internetwork. IP is described in RFC 791, which was
prepared for the Defense Advanced Research Projects Agency (DARPA) in September 1981.
IP provides for the transmission of blocks of data, called datagrams or packets, from a
source to a destination. The sources and destinations are identified by 32-bit IP addresses.
The source and destination devices are workstations, servers, printers, and routers. The
CCDA candidate must understand IPv4 logical address classes and assignment. The IPv4
protocol also provides for the fragmentation and reassembly of large packets for transport
over networks with small maximum transmission units (MTU). The CCDA candidate must
have a good understanding of this packet fragmentation and reassembly.
Appendix C, “OSI Model, TCP/IP Architecture, and Numeric Conversion,” provides an
overview of the TCP/IP architecture and how it compares with the OSI model. It also re-
views binary numbers and numeric conversion (to decimal), which is a skill needed to un-
derstand IP addresses and subnetting.
The best way to understand IPv4 is to know the IPv4 header and all its fields. Segments
from TCP or the User Datagram Protocol (UDP) are passed on to IP for processing. The IP
header is appended to the TCP or UDP segment. The TCP or UDP segment then becomes
the IP data. The IPv4 header is 20 bytes in length when it uses no optional fields. The IP
header includes the addresses of the sending host and destination host. It also includes the
upper-layer protocol, a field for prioritization, and a field for fragmentation. Figure 8-1
shows the IP header for mat.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
Version
IHL
Type of Service
To t a l L e n g t h
Identification
flags
Fragment Offset
Time to Live
Protocol
Header Checksum
Source Address
Destination Address
IP Options Field
Padding
Figure 8-1
IP Header
The following is a description of each field in the IP header:
Version:
This field is 4 bits in length. It indicates the IP header's format, based on the
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