ZyXEL Communications P-202 User Manual page 45

Prestige 201 ISDN Access Router
DNS Server Address
DNS (Domain Name System) is for mapping a domain name to its corresponding IP address and vice versa,
e.g., the IP address of www.zyxel.com is 204.217.0.2. The DNS server is extremely important because
without it, a user must know the IP address of a machine before s/he can access it. The DNS server addresses
that you enter in the DHCP setup are passed to the client machines along with the assigned IP address and
subnet mask.
There are two ways that an ISP disseminates the DNS server addresses. The first is for an ISP to tell a
customer the DNS server addresses, usually in the form of an information sheet, when s/he signs up. If your
ISP does give you the DNS server addresses, enter them in the DNS Server fields in DHCP Setup.
Some ISP's choose to pass the DNS servers using the DNS server extensions of PPP IPCP (IP Control
Protocol) after the connection is up. If your ISP did not give you explicit DNS servers, chances are the DNS
servers are conveyed through IPCP negotiation. The Prestige supports the IPCP DNS server extensions
through the DNS proxy feature.
If the Primary and Secondary DNS Server fields in DHCP Setup are not specified, i.e., left as 0.0.0.0, the
Prestige tells the DHCP clients that it itself is the DNS server. When a workstation sends a DNS query to the
Prestige, the Prestige forwards the query to the real DNS server learned through IPCP and relays the response
back to the workstation.
Please note that DNS proxy works only when the ISP uses the IPCP DNS server extensions. It does not
mean you can leave the DNS servers out of the DHCP setup under all circumstances. If your ISP gives you
explicit DNS servers, make sure that you enter their IP addresses in the DHCP Setup menu. This way, the
Prestige can pass the DNS servers to the workstations and the workstations can query the DNS server directly
without the Prestige's intervention.
Internet Access 3-3