Table of Contents
Advertisement
APPENDIX
There are two basic concepts that are needed for a basic understanding of networking, IP
addresses and subnet masks.
corresponding parts or sub-addresses, the first part identifying the network and the second part
identifying the hosts on the network. An imaginary boundary separates the first part from the
second. This imaginary boundary is marked by way of the subnet mask. The subnet mask is
another 32-bit binary number the acts like a filter on the IP address. When a subnet mask has a bit
set to 1, the corresponding bit in the IP address is part of the network address. A subnet is
classified as either a class A, class B, or class C network. The following table shows the common
subnet mask classes:
Class
Network Portion
A
11111111
B
11111111 11111111
C
11111111 11111111 11111111
For example, if you have an IP address of 169.254.1.1 and a subnet mask of 255.255.0.0, then the
first 16-bits of the 32-bit IP address identify the network.
10101001 11111110 00000001 00000001
11111111 11111111 00000000 00000000
16
There are 2
(65,536) addressable hosts in this example and 169.254 is the network. There is one
last piece of information that is needed here. Subnet masks are not shipped around in the IP
packet, the packet only contains the 32-bit IP address of the destination. So without this valuable
piece of information devices have no idea what portion of the IP address is part of the network and
which is part of the host address. How does data know where it is supposed to go? IP systems
developed a unique form of logic to make this determination. Class A network addresses always
have the first bit of their IP address set to 0. Class B network addresses always have their first bit
set to 1 and their second bit set to 0. Class C network addresses always have their first two bits
set to 1 and the third bit set to 0. By examining these first bits of the IP address a device can
determine what subnet mask should be applied to the IP address and determine where to route the
data.
The following is a synopsis of an Internet Draft
zeroconf-ipv4-linklocal-05.txt) that describes how Microsoft and Apple operating systems react
when a DHCP server is not found on the network.
configuration information to operate on a network. Those configurations are an IP address, a
subnet mask, and possibly a gateway address. A DHCP server will automatically assign this
configuration information to a computer on a network or an operator is required to manually input
these configurable items. When a computer is brought online and a DHCP server is not accessible
(i.e. server is down or the computer is not plugged into the network) Microsoft and Apple operating
systems will default to an IP address and subnet mask of 169.254.x.x and 255.255.0.0
(169.254/16).
IP addresses are 32-bit binary numbers that have two
Host Portion
00000000 00000000 00000000
00000000 00000000
00000000
(http://www.ietf.org/internet-drafts/draft-ietf-
In general, a computer needs certain
IP address
Subnet Mask
Advertisement
Table of Contents
