![]() 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. I'd suppose there were NAT rules, so even packets sent from the interface-state-independent address got src-nated to the address attached to the WAN interface when routed out through that interface.127.0.0.1. So the question is what traffic you were torching in the latter case and whether there were any NAT rules configured. What torch shows depends on the addresses in the packets, not on what address is attached to the interface. Even if the address fits into a subnet attached to some physical interface, Mikrotik will not respond ARP requests about that address even if they come through that physical interface. So a neighbor router only knows how to send a packet to that address if it has a route to it. To the second question - the interface-state-independent address only exists in the 元 domain. If there are no alternative paths in your network, there is no point in using interface-state-independent addresses. And you need that an outage of any link does not invalidate an IP address used by the dynamic routing protocols to talk to the router. ![]() It makes sense to use an "interface-state-independent" address in mesh topology networks with dynamic routing protocols, where you require path redundancy - if there is an outage on a link between nodes, the traffic gets routed via some other path instead. So the "interface-state-independent" address exists, but it is not attached to a loopback interface as such. ![]() ![]() So when you want to configure an IP address that will not depend on a state of any physical interface, you have to create a bridge interface with no member ports and attach the address to it. There is a "real" loopback interface on Mikrotik too, but it is not made visible in the RouterOS configuration. So on bare Linux systems, long before bridges were implemented, people started attaching addresses they needed to stay active no matter what to a loopback interface, and from there the shortcut "loopback address" comes. Addresses attached to a physical interfaces become inactive if the interface they are attached to goes down. The idea behind a "loopback address" is that this address is always active on the device. To the first question, I'd start from the fact that the popular name "loopback address" is technically wrong - it is a shortcut obfuscating the actual point. ![]()
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