Appendix A IP Addresses and Subnetting
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You can obtain your IP address from the IANA, from an ISP, or it can be assigned from a private network.
If you belong to a small organization and your Internet Access is through an ISP, the ISP can provide you
with the Internet addresses for your local networks. On the other hand, if you are part of a much larger
organization, you should consult your network administrator for the appropriate IP addresses.
Regardless of your particular situation, do not create an arbitrary IP address; always follow the guidelines
above. For more information on address assignment, please refer to RFC 1597, Address Allocation for
Private Internets and RFC 1466, Guidelines for Management of IP Address Space.
IP A d dre ss C o nflic ts
Each device on a network must have a unique IP address. Devices with duplicate IP addresses on the
same network will not be able to access the Internet or other resources. The devices may also be
unreachable through the network.
C o nflic ting C o m p ute r IP A dd re sse s Exa m p le
More than one device can not use the same IP address. In the following example computer
A
has a
static (or fixed) IP address that is the same as the IP address that a DHCP server assigns to computer
B
which is a DHCP client. Neither can access the Internet. This problem can be solved by assigning a
different static IP address to computer
A
or setting computer
A
to obtain an IP address automatically.
Fig ure 66
Conflicting Computer IP Addresses Example
C o nflic ting Ro ute r IP A d d re sse s Exa m p le
Since a router connects different networks, it must have interfaces using different network numbers. For
example, if a router is set between a LAN and the Internet (WAN), the router’s LAN and WAN addresses
must be on different subnets. In the following example, the LAN and WAN are on the same subnet. The
LAN computers cannot access the Internet because the router cannot route between networks.
