Private Ip Addresses - ZyXEL Communications ZYWALL 2 PLUS User Manual

Chapter 6 LAN Screens
Where you obtain your network number depends on your particular situation. If the ISP or
your network administrator assigns you a block of registered IP addresses, follow their
instructions in selecting the IP addresses and the subnet mask.
If the ISP did not explicitly give you an IP network number, then most likely you have a single
user account and the ISP will assign you a dynamic IP address when the connection is
established. If this is the case, it is recommended that you select a network number from
192.168.0.0 to 192.168.255.0 and you must enable the Network Address Translation (NAT)
feature of the ZyWALL. The Internet Assigned Number Authority (IANA) reserved this block
of addresses specifically for private use; please do not use any other number unless you are
told otherwise. If you select 192.168.1.0 as the network number; it covers 254 individual
addresses, from 192.168.1.1 to 192.168.1.254 (zero and 255 are reserved). In other words, the
first three numbers specify the network number while the last number identifies an individual
computer on that network.
Once you have decided on the network number, pick an IP address that is easy to remember,
for instance, 192.168.1.1, for your ZyWALL, but make sure that no other device on your
network is using that IP address.
The subnet mask specifies the network number portion of an IP address. Your ZyWALL will
compute the subnet mask automatically based on the IP address that you entered. You don't
need to change the subnet mask computed by the ZyWALL unless you are instructed to do
otherwise.

6.2.1 Private IP Addresses

Every machine on the Internet must have a unique address. If your networks are isolated from
the Internet, for example, only between your two branch offices, you can assign any IP
addresses to the hosts without problems. However, the Internet Assigned Numbers Authority
(IANA) has reserved the following three blocks of IP addresses specifically for private
networks:
• 10.0.0.0
• 172.16.0.0 — 172.31.255.255
• 192.168.0.0 — 192.168.255.255
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.
124
— 10.255.255.255
ZyWALL 2 Plus User's Guide