ZyXEL Communications USG FLEX H Series User Manual page 101

For example, if the Zyxel Device gets a packet with a destination address of 100.100.25.25, it routes the
packet to interface lan1. If the Zyxel Device gets a packet with a destination address of 200.200.200.200,
it routes the packet to interface wan1.
In most interfaces, you can enter the IP address and subnet mask manually. In PPPoE interfaces,
however, the subnet mask is always 255.255.255.255 because it is a point-to-point interface. For these
interfaces, you can only enter the IP address.
In many interfaces, you can also let the IP address and subnet mask be assigned by an external DHCP
server on the network. In this case, the interface is a DHCP client. Virtual interfaces, however, cannot be
DHCP clients. You have to assign the IP address and subnet mask manually.
In general, the IP address and subnet mask of each interface should not overlap, though it is possible for
this to happen with DHCP clients.
In the example above, if the Zyxel Device gets a packet with a destination address of 5.5.5.5, it might
not find any entries in the routing table. In this case, the packet is dropped. However, if there is a default
router to which the Zyxel Device should send this packet, you can specify it as a gateway in one of the
interfaces. For example, if there is a default router at 200.200.200.100, you can create a gateway at
200.200.200.100 on ge2. In this case, the Zyxel Device creates the following entry in the routing table.
Table 58 Example: Routing Table Entry for a Gateway
IP ADDRESS(ES)
0.0.0.0/0
The gateway is an optional setting for each interface. If there is more than one gateway, the Zyxel
Device uses the gateway with the lowest metric, or cost. If two or more gateways have the same metric,
the Zyxel Device uses the one that was set up first (the first entry in the routing table). In PPPoE interfaces,
the other computer is the gateway for the interface by default. In this case, you should specify the
metric.
If the interface gets its IP address and subnet mask from a DHCP server, the DHCP server also specifies
the gateway, if any.
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 on 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 Zyxel Device, 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.
Chapter 7 Interfaces
DESTINATION
200.200.200.100
USG FLEX H Series User's Guide
101