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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Domain Name Resolution
The domain name system can be used by contacting the name servers one at a time, or by asking the
domain name system to do the complete name translation. The client makes a query containing the
name, the type of answer required, and a code specifying whether the domain name system should do
the entire name translation, or simply return the address of the next DNS server if the server receiving
the query cannot resolve the name.
When a DNS server receives a query, it checks to see if the name is in its subdomain. If it is, the server
translates the name and appends the answer to the query, and sends it back to the client. If the DNS
server cannot translate the name, it determines what type of name resolution the client requested. A
complete translation is called recursive resolution and requires the server to contact other DNS servers
until the name is resolved. Iterative resolution specifies that if the DNS server cannot supply an
answer, it returns the address of the next DNS server the client should contact.
Each client must be able to contact at least one DNS server, and each DNS server must be able to
contact at least one root server.
The address of the machine that supplies domain name service is often supplied by a DHCP or BOOTP
server, or can be entered manually and configured into the operating system at startup.
DHCP Servers
The Dynamic Host Configuration Protocol (DHCP) is used to dynamically assign a TCP/IP network
configuration to network devices and computers on the network. It also ensures that IP address
conflicts do not occur.
IP addresses are assigned from a pool of free addresses. Each IP address assigned has a ‘lease’ and a
‘lease expiration period’. The lease must be periodically renewed. If the lease is expires, the IP address
is returned to the pool of available IP addresses.
Usually, it is a network policy to assign the same IP address to a given network device or computer
each time.
If the IP address lease expires, the network device sends a message to the DHCP server requesting a
lease renewal. The DHCP server can send an acknowledgement containing a new lease and updated
configuration information.
If an IP address lease cannot be renewed, the network device or computer sends a request to all local
DHCP servers attempting to renew the lease. If the DHCP returns a negative acknowledgement, the
network device must release its TCP/IP configuration and reinitialize.
When a new TCP/IP configuration is received from a DHCP server, the network device checks for a
possible IP address conflict by sending an Address Resolution Protocol (ARP) request that contains its
new IP address.
For two DHCP servers to communicate across different subnets, the BOOTP/DHCP Relay of the
DES-3326 must be used. The DHCP servers are identified by IP addresses.
IP Routing
IP handles the task of determining how packets will get from their source to their destination. This
process is referred to as routing.
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