Which addresses represent valid local loopback addresses check all that apply

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.

An IP address is an identifier for a device connected to a network using TCP/IP - a protocol that routes network traffic based on the IP address of its destination. IP addresses can either be 32-bit IPv4 addresses consisting of four base-10 numbers separated by periods representing eight digit binary (base-2) numbers called “octets” (i.e. 0.0.0.0 to 255.255.255.255), or 128-bit IPv6 addresses consisting of eight hexadecimal (base-16) numbers separated by colons representing sixteen digit binary (base-2) numbers (i.e. 0000:0000:0000:0000:0000:0000:0000:0000 to FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF where consecutive groups of four zeroes are replaced by a double colon).

When the Internet first became popular, IPv4, with its 32-bit addresses, offered 232, or roughly 4.3 x 109 unique addresses. As the number of Internet-connected devices began to grow significantly, people worried that the IPv4 protocol would not contain enough addresses to meet the growing demand for new unique addresses - this is why IPv4 will eventually be replaced by IPv6 on a large scale (IPv6 already officially launched in June 2012), which contains 2128 or roughly 3.4 x 1038 unique addresses.

The Dynamic Host Configuration Protocol (DHCP), which runs on special devices (usually routers) allows for the assigning of IP addresses within a local area network (LAN). DHCP assigns IP addresses on a temporary “lease” basis; once a device’s IP address lease expires, a DHCP server will assign it a new (potentially different) one. IP addresses automatically assigned by a DHCP server are therefore referred to as “dynamic IP addresses”, as a device with a DHCP-assigned IP address may eventually receive an IP different from its original one.

DHCP servers will not assign devices just any IP address in the maximum range of IPv4 addresses (0.0.0.0 to 255.255.255.255), as certain IP addresses are reserved for special purposes. Such addresses include:

• 0.0.0.0 – Represents the “default” network, i.e. any connection
• 255.255.255.255 – Represents the broadcast address, or place to route messages to be sent to every device within a network
• 127.0.0.1 – Represents “localhost” or the “loopback address”, allowing a device to refer to itself, regardless of what network it is connected to
• 169.254.X.X – Represents a “self-assigned IP address”, which a device will assign itself if it is unable to receive an IP address from a DHCP server

Users’ DHCP-assigned IP addresses on a LAN are not the same as their “external” or Internet IP address. This address will be the same for all users connected to a DHCP server, which itself receives an IP address from the Internet Service Provider (ISP) it is connected to.

As IP addresses can be used as unique identifiers for users’ machines (and subsequently the users themselves), knowledge of a malicious user’s external Internet IP address can allow law enforcement officials to block, locate, and eventually arrest him or her. As a result, the more advanced attack tools and hackers will employ anonymization techniques - such as the use of proxy servers, VPNs, or a routing network like Tor or I2P - that can make it seem like they are using a different IP address other than their own, located somewhere else in the world. An attack tool called Low Orbit Ion Cannon (LOIC) became infamous for not hiding its users’ IP addresses; this resulted in the arrest of various LOIC users around the world for their participation in distributed denial-of-service (DDoS) attacks.

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