They’re are many important protocols that you need knowledge of if you’re troubleshooting complicated networks. First of all there’s TCP, IP and UDP plus a host of application protocols such as DHCP and DNS. Any of these could be an issue if you’re having problems with a network. However often the most difficult to troubleshoot and indeed to understand are the lower level protocols such as ARP. If you don’t have some understanding of these it can be extremely confusing to understand how they interact.

The address resolution protocol often sits in the background happily resolving addresses, however if you get issues it can cause some very difficult problems. If you’re working on some sort of complicated network such as a residential proxy set up or ISP like this, there will be all sorts of hardware resolution requests taking place on both local and remote networks.

Both logical and physical addresses are used for intercommunication on a network. The use of logical addresses permits communication among a wide range of networks and not directly connected devices. The use of physical addresses assists in communication on a single network segment for devices that are directly connected to each other with a switch. In the majority of cases, these two kinds of addressing must collaborate in order for communication to happen.

Consider a scenario where you want to communicate with a device on your network. This device may be a server of some kind or simply another work- station you have to share files with. The application you are utilizing to launch the communication is already aware of the IP address of the remote host (by means of DNS, addressed elsewhere), meaning the system should have all it needs to build the layer 3 through 7 information of the packet it wishes to transmit.

The only piece of info it needs at this point is the layer 2 data link information consisting of the MAC address of the intended host. MAC addresses are required for the reason that a switch that interconnects devices on a network uses a Content Addressable Memory (CAM) table, which specifies the MAC addresses of all devices plugged into each of its ports. When the switch acquires traffic destined for a specific MAC address, it makes use of this table to know through which port to send the traffic.
If the destination MAC address is not known, the transmitting device will definitely first check for the address in its cache; if it is not there, then this must be resolved by means of supplementary communicating on the network.

The resolution procedure that TCP/IP networking (along with IPv4) uses to resolve an IP address to a MAC address is referred to as the Address Resolution Prrotocol (ARP), which is defined in RFC 826. The ARP resolution process uses only two packets: an ARP request and an ARP response.


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