ARP,DHCP and DNS Concepts
1. Address Resolution Protocol (ARP)
(a) HARDWARE ADDRESSING
(b) HOW ARP RESOLVES MEDIA ACCESS CONTROL
(c) ARP CACHE
(d) ARP COMMAND
1- To view the arp cache, type arp –a at a command prompt.
2. Dynamic Host Configuration Protocol
1- Address resolution protocol (arp)." arp resolves IP addresses used by TCP/IP-based software to media access control addresses used by lan hardware.
2- Arp provides the following protocol services to hosts located on the same physical network.
3- Media access control addresses are obtained by using a network broadcast request in the form of the question "what is the media access control address for a device that is configured with the enclosed IP address?"
4- When an arp request is answered, both the sender of the ARP reply and the original arp requester record each other's IP address and media access control address as an entry in a local table called the arp cache for future reference.
(a) HARDWARE ADDRESSING
1- Hardware built for use on LANs must contain a unique address programmed into the device by the manufacturer. For Ethernet and token ring LAN hardware, this address is known as a media access control address.
2- Each media access control address identifies the device within its own physical network with a 6-byte number programmed into read-only memory (rom) on each physical hardware device, such as a network adapter. media access control addresses are typically displayed in hexadecimal (for example, 00-aa-00-3f-89-4a).
3- Authority and registration of media access control addresses are overseen by the institute of electrical and electronics engineers (IEEE). Currently, the IEEE registers and assigns unique numbers for the first three bytes of the media access control address to individual manufacturers. Each manufacturer can then assign the last three bytes of the media access control address to individual network adapters.
(b) HOW ARP RESOLVES MEDIA ACCESS CONTROL
(i) Host A is assigned the IP address of 10.0.0.99 and Host B is assigned the IP address of 10.0.0.100.
(ii) When Host A tries to communicate with Host B, the following steps resolve Host B’s software-assigned address (10.0.0.100) to Host B’s hardware-assigned media access control address.
(iii) Based on the contents of the routing table on Host A, IP determines that the forwarding IP address to be used to reach Host B is 10.0.0.100. Host A then checks its own local arp cache for a matching hardware address for Host B.
(iv) If Host A finds no mapping in the cache, it broadcasts an arp request frame to all hosts on the local network with the question "what is the hardware address for 10.0.0.100?" both hardware and software addresses for the source, Host A, are included in the arp request.
(v) Each host on the local network receives the arp request and checks for a match to its own IP address. If a host does not find a match, it discards the ARP request.
(vi) Host B determines that the IP address in the ARP request matches its own IP address and adds a hardware/software address mapping for Host A to its local ARP cache.
(vii) Host B sends an ARP reply message containing its hardware address directly back to Host A. When Host A receives the ARP reply message from Host B, it updates its ARP cache with a hardware/software address mapping for Host B.
(viii) Once the media access control address for Host B has been determined, Host A can send IP traffic to Host B by addressing it to Host B’s media access control address.
(c) ARP CACHE
(i) To minimize the number of broadcasts, ARP maintains a cache of IP address–to–media access control address mappings for future use.
(ii) The ARP cache can contain both dynamic and static entries.
(iii) Dynamic entries are added and removed automatically over time
(iv) Static entries remain in the cache until the computer is restarted.
(v) Each dynamic ARP cache entry has a potential lifetime of 10 minutes.
(vi) New entries added to the cache are time stamped. if an entry is not reused within 2 minutes of being added, it expires and is removed from the ARP cache.
(vii) If an entry is used, it receives two more minutes of lifetime. If an entry keeps getting used, it receives an additional two minutes of lifetime up to a maximum lifetime of 10 minutes.
(d) ARP COMMAND
2- To view arp command-line options, type arp /? at a command prompt
3- There is a separate arp cache for each network adapter.
4- To display the arp cache tables for all interfaces, type: arp –a
5- To display the arp cache table for the interface that is assigned the IP address 10.0.0.99, type: arp -a -n 10.0.0.99
6- To add a static arp cache entry that resolves the IP address 10.0.0.80 to the physical address 00-aa-00-4f-2a-9c, type: arp -s 10.0.0.80 00-aa-00-4f-2a-9c
2. Dynamic Host Configuration Protocol
1- It gives IP addresses automatically to the clients who are requesting for a dynamic IP address
2- DHCP uses a client/server model where the DHCP server maintains centralized management of IP address that are used on the network.
3- DHCP provides safe, reliable and simple TCP/IP network configuration.
4- DHCP prevents address conflicts and helps conserve the use of client IP addresses on the network
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COMPUTING KNOWLEDGE
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