ZyXEL Communications ZyWall USG 50-H Series, User Manual

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Chapter 8 Interface
ZyWALL USG 50-H User’s Guide
215
DHCP Settings
Dynamic Host Configuration Protocol (DHCP, RFC 2131, RFC 2132) provides a way to
automatically set up and maintain IP addresses, subnet masks, gateways, and some network
information (such as the IP addresses of DNS servers) on computers in the network. This
reduces the amount of manual configuration you have to do and usually uses available IP
addresses more efficiently.
In DHCP, every network has at least one DHCP server. When a computer (a DHCP client)
joins the network, it submits a DHCP request. The DHCP servers get the request; assign an IP
address; and provide the IP address, subnet mask, gateway, and available network information
to the DHCP client. When the DHCP client leaves the network, the DHCP servers can assign
its IP address to another DHCP client.
In the ZyWALL, some interfaces can provide DHCP services to the network. In this case, the
interface can be a DHCP relay or a DHCP server.
As a DHCP relay, the interface routes DHCP requests to DHCP servers on different networks.
You can specify more than one DHCP server. If you do, the interface routes DHCP requests to
all of them. It is possible for an interface to be a DHCP relay and a DHCP client
simultaneously.
As a DHCP server, the interface provides the following information to DHCP clients.
• IP address - If the DHCP client’s MAC address is in the ZyWALL’s static DHCP table, the
interface assigns the corresponding IP address. If not, the interface assigns IP addresses
from a pool, defined by the starting address of the pool and the pool size.
The ZyWALL cannot assign the first address (network address) or the last address
(broadcast address) in the subnet defined by the interface’s IP address and subnet mask.
For example, in the first entry, if the subnet mask is 255.255.255.0, the ZyWALL cannot
assign 50.50.50.0 or 50.50.50.255. If the subnet mask is 255.255.0.0, the ZyWALL cannot
assign 50.50.0.0 or 50.50.255.255. Otherwise, it can assign every IP address in the range,
except the interface’s IP address.
If you do not specify the starting address or the pool size, the interface the maximum range
of IP addresses allowed by the interface’s IP address and subnet mask. For example, if the
interface’s IP address is 9.9.9.1 and subnet mask is 255.255.255.0, the starting IP address
in the pool is 9.9.9.2, and the pool size is 253.
• Subnet mask - The interface provides the same subnet mask you specify for the interface.
See IP Address Assignment on page 213 .
• Gateway - The interface provides the same gateway you specify for the interface. See IP
Address Assignment on page 213 .
Table 77 Example: Assigning IP Addresses from a Pool
START IP ADDRESS POOL SIZE RANGE OF ASSIGNED IP ADDRESS
50.50.50.33 5 50.50.50.33 - 50.50.50.37
75.75.75.1 200 75.75.75.1 - 75.75.75.200
99.99.1.1 1023 99.99.1.1 - 99.99.4.255
120.120.120.100 100 120.120.120.100 - 120.120.120.199