DNS information is configurable only on the primary server, and not on the backup server. A caching DNS server other than the primary and secondary DNS servers for this domain or network can cache record lookups for the TTL (Time To Live) period. During the TTL period, a caching DNS server does not poll the primary or secondary DNS servers for repeated lookups of the same record.
ARP.
ARP stands for Address Resolution Protocol. Its only task is to obtain the hardware addresses of TCP/IP hosts on broadcast-based networks. ARP uses a local broadcast of the destination IP address to obtain the hardware address of the host. ARP uses only local broadcasts to a hosts or a gateway. As soon as the hardware address is obtained, the IP address and the hardware address are stored in the ARP cache. This cache is checked for IP/Hardware address mapping before an ARP broadcast is initiated. If an ARP request is initiated from the host to a destination host on a remote network, the ARP request is sent to a router or gateway. This can only be done if the router/gateway is able to forward datagrams to the destination host's network.
DHCP.
DHCP stands for Dynamic Host Configuration Protocol, and is used to centrally manage TCP/ IP configurations of client nodes. If you've got more than a handful of computers to manage, then DHCP can help to save a great deal of time and trouble in setting up and administering a TCP/ IP network. .
DHCP offers the following features: .
* it allows you to define "pools" of TCP/ IP addresses, which are then allocated to client PCs by the server. These pools are called scopes in DHCP terminology. .
* Not only are the TCP/ IP addresses handed out, so are all the related configuration settings like the subnet mask, default router, DNS server, that are required to make TCP/ IP work correctly. .
* DHCP works across most TCP/ IP routers and assigns IP's according to the subnet the request came from.
