![]() ![]() In most cases, localhost is just an acronym that, by default, refers to 127.0.0.1. How Does a 127.0.0.1 Loopback Work? What Is Localhost? This aspect of loopback helps to ensure network security. As a result, if a data packet accidentally gets outside the local node, it will not reach the computer. In that case, it should discard the packet without logging information. Suppose any public switch, router, or gateway receives a packet that someone has addressed to a loopback IP address. When establishing an IPv4 connection to 127.0.0.1, a subnet mask of 255.0.0.1 is generally assigned. Usually, system administrators and application developers use 127.0.0.1 to test applications. The main difference is that connecting to 127.0.0.1 does not use the LAN interface hardware. How Does 127.0.0.1 Work?Įstablishing a network connection to the loopback address 127.0.0.1 is almost identical to any remote computer or device on the network. Therefore, the protocol does not guarantee delivery, data integrity, or correct data consistency instead, relying on the Transmission Control Protocol (TCP) to solve these problems. IPv4 is considered a connectionless protocol intended for use in Ethernet networks. Although IPv6 is gradually replacing IPv4, its adoption is still under development. ![]() IETF RFC 791, finalized in September 1981, is the current definition of the protocol. In IPv4, 127.0.0.1 is the most widely used worldwide. IP (Internet Protocol) is a set of rules that provides a standardized method of addressing and communication between computers and other network devices. Despite being the most common and well-known address, 127.0.0.1 is just one address in a large block (127.0.0.0 – 127.255.255.255) reserved for backhaul in RFC 6890. Thus, routers that receive traffic destined for 127.0.0.1 must drop the packets, which ensures that no traffic destined solely for the host computer reaches the Internet. Such addresses MUST NOT appear outside the host”. RFC 1122 says – “Internal loopback host address. When you send it, instead of throwing it to the local network or the Internet, it simply “loops” on itself, and the computer that sent the packet becomes the recipient. A host loop means no data packet addressed to 127.0.0.1 should ever leave the computer (host). As the name implies, the application for this address is local – to establish an IP connection with your computer. ![]() 127.0.0.1 is a loopback Internet Protocol address called “localhost”. However, what is it, and why is it so famous? Let’s get to the bottom of it now. It appears in memes, on t-shirts as well as in tech documentation. ![]() And this is the case where loopback address for router ID in OSPF is beneficial.127.0.0.1. But (!!) if any other router ( A, B, C or D) will have one (or more) of its interface(s) down, then if the router ID was not "set deterministically" - new advertisement will have to be sent onto the network, which will affected the overall bandwidth of it. On the other hand, if the router ID was set "deterministically" by configuring it to be the loopback address (or if there is any loopback address in the OSPF process), that will never go down (unless of course, the entire router/OSPF process will go down), then if any one of the interfaces of the router goes down, the router ID won't be affected, therfor no multicast OSPF "new router ID" messages will be sent onto the network.Ĭonsidering the above topology, in case router E (or more precisely its only interface) goes down, then anyway, when it will go up again it will still advertise its router ID "all over again". So, on any link failure for a specific router - if the logic of the router ID selection is still set on the "highest IP address" and there is no loopback address configured as well in the OSPF process (or there is no loopback address in the router at all) - then this link failure will trigger a new router ID selection procedure "within" the router and, perhaps more importantly, will obligate this router to advertise its "newly elected" router ID, meaning send OSPF messages again onto the network. This ID is chosen among ALL available interfaces on a given router UNLESS explicitly configured otherwise. As others mentioned, every OSPF router chooses a router ID. Adding to Maupin great answer I would further say that the (wise) choice of the router ID to be the loop back interface will be more "powerful" upon link failure scenarios. ![]()
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