To keep track of the duration of IP address assignment, a DHCP server uses the concept of leasing. This makes the DHCP server handle the sub-net that has no server available and thus there is no need to setup a server per sub-net. Thus the relay agent is for relaying packets between servers and clients. The relay agent usually keeps the IP address of the DHCP server. It is send as a unicast transmission to the DHCP server (which may be on another network) by the relay agent. Hence a DHCP relay agent is needed to forward the DHCP DISCOVER broadcast packet from a newly booted machine. DHCP server is not reachable by broad- casting from a different network. It allows manual and dynamic IP address assignment to computers that requests for that. It is an extension of BOOTP, the previous IP allocation specification. Dynamic Host Control Protocol (DHCP) is defined in RFC 21. DHCP stands for ‘Dynamic Host Configuration Protocol’ and is a way by which networked computers get their TCP/IP networking settings from a central server. To allow for changes in mapping, especially when network card breaks down, and is replaced with a new one, entries in ARP cache should time out after few minutes. Another optimization is to have every computer broadcast its mapping when it boots, in the form of an ARP looking for its won address. In fact all machines on LAN1 can enter this mapping information on A into their ARP cache. Usually, A would include its IP-to-MAC address mapping in the ARP packet, thus informing B of its mapping. So if in a short period of time, if A wants to communicate with B, it refers to the local ARP cache, eliminating a second broadcast. Once computer A gets the ARP reply from B, it stores that IP- to-MAC address mapping of B in a local cache. Some optimizations are possible with ARP.
Thus ARP works by this request and reply approach. Only computer B would respond with its MAC address MAC B. This broadcast would reach on all computers in LAN1.
it broadcasts a packet asking, “Who own IP address IP B?”.
0 kommentar(er)
