Which of the following ip addresses can be used as (a) loop-back addresses?

An address that sends outgoing signals back to the same computer for testing. In a TCP/IP network, the loopback IP address is 127.0.0.1, and pinging this address will always return a reply unless the firewall prevents it. The loopback address allows a network administrator to treat the local machine as if it were a remote machine. See ping, loopback plug and localhost.

A loopback address has been built into the IP domain system in order to allow for a device to send and receive its own data packets.

Loopback addresses can be useful in various kinds of analysis like testing and debugging, or in allowing routers to communicate in specific ways.

A simple way of describing how using a loopback address works is that a data packet will get sent through a network and routed back to the same device where it originated.

In IPv4, 127.0.0.1 is the most commonly used loopback address, however, this can range be extended to 127.255.255.255.

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Techopedia Explains Loopback Address

The Loopback Interface

The loopback address is a vital component of what IT experts call a loopback interface. The loopback interface refers to the overall system by which network engineers can self-reference a device, or “ping” a device by sending its data packets back to itself.

A loopback interface helps to solve some router problems and implement some kinds of testing. For example, a paperclip test will utilize terminal emulator application methodology to verify some type of network connectivity. Other kinds of testing look at how routers are set up and how they talk to each other, and what can be done to evaluate the functionality of a specific part of a network.

The loopback address is also useful in Border Gateway Protocol scenarios that connect routers through inter-domain routing between autonomous systems. The autonomous system, as its own internal network, has its own protocols, to which a loopback address test can be a remedy for certain problems with network verification.

Different Loopback Addresses for IPv4 and IPv6 Domain Addressing Systems

The loopback address also looks different in IPv4 than it does in IPv6. In recent years, the Internet Protocol system has been retrenched to allow for a greater number of IP addresses. In the old IPv4 system, the loopback address was 127.0. 0.1. The syntax of the new loopback address in IPv6 is simpler: it's ::1.

Juniper Networks describes some of the corresponding change this way: “Most IP implementations support a loopback interface (lo0) to represent the loopback facility. Any traffic that a computer program sends on the loopback network is addressed to the same computer. The most commonly used IP address on the loopback network is 127.0.0.1 for IPv4 and ::1 for IPv6. The standard domain name for the address is localhost.”

Benefits of Using a Loopback Address

The benefits of using a loopback address to ping a given network device are useful in understanding how this kind of implementation works.

One of the biggest benefits of using the loopback address is that it is “always up” and available. Even in various sorts of system failures or network downtime, the loopback interface is still in place, allowing engineers to verify whether a given device connection is solvent or not.

Another key benefit is that the loopback address never changes depending on IP setups. It's a universal and constant part of the availability of systems to evaluate their connections.

Users can also implement stateless firewall filters in a loopback interface and loopback address use case.

With all of this in mind, the loopback address is a simple but critical part of the network engineer’s toolkit.

IPv4 has special reserved addresses called as loopback addresses. An entire Class A network itself is reserved as loopback network. The loopback network in IPv4 is 127.0.0.0 with a subnet mask of 255.0.0.0.

So, the range of loopback addresses in IPv4 is from 127.0.0.1 to 127.255.255.254. As mentioned in previous lessons, 127.0.0.0 is the network address and 127.255.255.255 is the directed broadcast address for 127.0.0.0 255.0.0.0 network.

Loopback IP addresses are managed by the TCP/IP protocol suite within the operating system. Loopback addresses mock TCP/IP Client/Server on the same machine. The loopback IP addresses are always available. Hence you can use the loopback IP addresses for TCP/IP troubleshooting purposes.

When any program/protocol sends data from a computer with any IPv4 loopback address as the destination address, the TCP/IP protocol stack on that computer process the traffic within itself without sending it to the network. In other words, if you ping to a loopback address, you get the reply from the TCP/IP protocol stack running on the same computer. Any data traffic sent to IPv4 loopback addresses from 127.0.0.1 to 127.255.255.254 as the destination IPv4 address will never appear on network.

The most widely used IPv4 loopback address is 127.0.0.1. Loopback address is 127.0.0.1 is mapped to hostname localhost internally. Other IPv4 addresses in 127.0.0.0 255.0.0.0 network (from 127.0.0.1 to 127.255.255.254) are also up and reachable. For example; below two screen shots show the output of ping command (from a Windows machine) from two IPv4 loopback addresses. Both screen shots show that ping command got reply from loopback addresses.

Below screenshot shows ping reply from localhost IPv4 loopback address, 127.0.0.1.

Below screenshot shows ping reply from a random IPv4 loopback address from 127.0.0.0 255.0.0.0 network. The random IPv4 loopback address selected is 127.10.100.50.

Network address and directed broadcast address of loopback address range are not reachable. Please check below ping results to 127.0.0.0 and 127.255.255.255, which shows "General failure" in Windows TCP/IP implementation.

Which of the following IP addresses Cannot be used as a loop back addresses?

Are all 127 addresses loopback?

What is the 127.0 0.1 address used for?

What is IPv6 loop back address?