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Appendix D: Network, Routing, and Firewall Basics
D.1 Network Basics
D.1.1 IP Addresses
With the number of TCP/IP networks interconnected across the globe, ensuring that
transmitted data reaches the correct destination requires each computer on the
Internet has a unique identifier. This identifier is known as the IP address. The
Internet Protocol (IP) uses a 32-bit address structure, and the address is usually
written in dot notation.
A typical IP address looks like this:
198.25.12.8
The 32 bits of the address are subdivided into two parts. The first part of the address
identifies the network, while the second part identifies the host node or station on
the network. How the address is divided depends on the address range and the
application.
The five standard IP address classes each have different methods to determine the
network and host sections of the address, which makes multiple hosts on a network
possible. TCP/IP software identifies each address class by reading a unique bit
pattern that precedes each address type. Once the address class has been
recognized, the software can then correctly determine the addresses’ host section.
With this structure, IP addresses can uniquely identify each network and node.
D.1.1.1 Net mask
With each address class, the size of the two subdivided parts (network address and
host address) is implied by the class. A net mask associated with an IP address can
also express this partitioning. A net mask 32-bit quantity yields the network address
when combined with an IP address. As an example, the net masks for Class A, B,
and C are 255.0.0.0, 255.255.0.0, and 255.255.255.0 respectively.
Instead of dotted-decimal notation, the net mask can also be written in terms of the
number of ones from the left. This number is added to the IP address, following a