What ARP does | Network Fundamentals Part 15

devices on a network commonly use both IP addresses and MAC addresses to communicate for how the devices know which IP addresses go with which MAC addresses this is possible due to a protocol called ARP and in this video we're going to see what it does and how it works think about the OSI model there are several layers which need to work together to get from one host to another frequently IP addresses at layer 3 are used to identify which device to send traffic to but there is also a layer 2 path to follow and devices at layer 2 use MAC addresses imagine that you have two hosts within the same subnet perhaps one is a web server and the other is a client the client knows the IP address of the web server and wants to start an HTTP session so the client builds a TCP segments and encapsulates it in a layer 3 header it needs to add a layer 2 header as well but what destination MAC address cannot put in this header right now it has no idea it simply does not know the destination MAC address and this is where address resolution protocol comes in the whole point of ARP is to find the MAC address that belongs with an IP address up works by sending a layer to broadcast message to the entire land this message is called an arp request the request contains the statement who is 1010 1021 tel [phone] for all the devices on the LAN will get this message they will look at the IP address in the request and see if it is their own for most of them this will not be their address so they will discard the message but if the device we're looking for gets the message it will then create an up response the response will contain its own IP address as well as its MAC address the response will be sent as a unicast message only to the host listed in the original request now that the client has its response it will put this information into its ARP cache this is a small table that contains all the IP 2 MAC address mappings that it has learned so far caching is done so there will be no need to send an ARP request for every single packet that it needs to send these entries aren't permanent though over time if the entry is not used it will be removed from the cache to give you a rough idea a modern version of Windows will store an up entry for somewhere between 15 and 45 seconds there are two reasons for this for one there is limited space in the ARP table clearing out stale entries helps keep the table small and organized for another devices sometimes change their IP addresses if the ARP cache entry had an indefinite lifetime one device may have trouble noticing another's IP address change we can see this from the Windows command line this here is its app cache you can see all sorts of entries in here now if we paint something new windows will need to send an ARP request to find the MAC address - IP mapping and this usually happens very quickly and if we look at the ARP case again we can see a new entry has been learned in addition to regular up there is also wrap and GARP wrap is reverse up and it does exactly what it sounds like if a device knows a MAC address and wants to find an IP it will send a wrap request and the Mac owner will send a wrap reply the really cool one is GARP or gratuitous up remember how I said before that IP addresses can change ideally we don't want to wait for the ARP cache entries to expire before we learn about these changes so if a device has good manners it will send a GARP message when it's changing its IP this is its way of announcing its new Mac - IP mapping - everything on the local network this isn't the only time this happens when a device first boots up it will also send a GARP message and this has two advantages firstly devices learn the IP to mock mapping for the new device without needing to send an ARP out secondly if the IP of the new device is already in use the device already using it can send a GARP reply this helps to prevent IP conflicts I hope this has all been making a sense you can find out if it is making sense to you by testing yourself out with these quiz questions it's nice to know how devices get IP to MAC address mappings but how do devices get their own IPS in the first place surely we can't be expected to manually assign IP addresses to every device on the network the good news is that we don't have to there is a service called DHCP which automatically allocates IPs and that's what we're going to have a look at in the next video I'll see you there

Channel: Network Direction

Views: 28,600

Keywords: Arp, Address resolution protocol, Network direction, Network fundamentals, Rarp, Garp, Mac, Ip, Request, Reply, Osi, tcp/ip, Resolution, Who has x.x.x.x? Tell y.y.y.y, Arp cache, Stale entry, Reverse arp, Gratuitous arp, Silent host, Ip conflict, mac address, how arp works, arp protocol, address resolution protocol (protocol)

Id: 98CAjGW0lzk

Length: 5min 35sec (335 seconds)

Published: Tue Aug 20 2019