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Chapter 10 Interface
ZyWALL USG 100/200 Series User’s Guide
267
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 265.
• Gateway - The interface provides the same gateway you specify for the interface. See IP
Address Assignment on page 265.
• DNS servers - The interface provides IP addresses for up to three DNS servers that
provide DNS services for DHCP clients. You can specify each IP address manually (for
example, a company’s own DNS server), or you can refer to DNS servers that other
interfaces received from DHCP servers (for example, a DNS server at an ISP). These other
interfaces have to be DHCP clients.
It is not possible for an interface to be the DHCP server and a DHCP client simultaneously.
Table 85 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