Difference between 127.0.0.1 and 0.0.0.0

In this article, we are going to understand the difference between 127.0.0.1 and 0.0.0.0 IP addresses. However, before trying to understand the difference between different IP addresses, we need to know some of the terminologies like:

1. What is Computer Network?
2. What is IP addresses?

What is Computer Network?
It is a collection of computers connected electronically that share data among themselves. A network system can be used as an information transfer conduit for facilitating the transfer of data between various types of equipment in addition to connecting computers and communication devices. Two of the most popular computer network architectures are IBM’s Systems Network Architecture (SNA) and ISO’s Open Systems Interconnection (OSI).

Types of Computer Networks are:

1. WAN(Wide Area Network)
2. MAN(Metropolitan Area Network )
3. LAN(Local Area Network)
4. PAN(Personal Area Network)
5. WLAN(Wireless Local Area Network)
6. SAN(Storage Area Network)
7. SAN(System Area Network)
8. CAN(Campus Area Network)

How does Computer Network work?
The data network transmits packets of data by means of ethernet cables and routers. Upon receiving the data, the router interprets it and sends it to the right area. The information on a network is divided into smaller portions called packets. As the packets are transmitted through cable, the data is loaded on your equipment by routers, switches, or servers. It can be nearly impossible to summarize all the functions that networks are capable of.

What is an IP address?
In a computer network that uses the Internet Protocol for communication, an IP address (Internet Protocol address) is a numerical label, 192.0.2.1. Two functions of an IP address are to identify hosts and to address locations. According to Internet Protocol version 4 (IPv4), IP addresses are 32-bit numbers. Due to the growing use of the Internet and the depletion of IPv4 addresses, a new version of the IP protocol (IPv6) was standardized in 1998. IPv6 was being deployed in the early 2000s.

For more understanding of IP address, you can check IP address.

Now, let us try to understand two special and reserved IP addresses, and these are

1. 127.0.0.1
2. 0.0.0.0

1. 127.0.0.1 IP Address: 
The IP address 127.0.0.1, also called loopback, is exclusively for localhost use. It is possible for computers to communicate with each other over an IP address, but this address cannot be used by computers to communicate with one another. A chance exists that the private IP address 192.168.1.115 is assigned to your computer so that it can connect to a router or networked device. A computer still has an alias of 127.0.0.1 in networking terms. Unlike loopback addresses, IP addresses are the connection between your computer and the network, not the connection between the computer and the loopback address. As an example: The web server on a particular operating system may declare 127.0.0.1 as the local hostname, so the pages can be run locally before they are deployed.

How 127.0.0.1 works?
IP addresses are always attached to TCP/IP application software messages. It can reduce network security by eliminating any loopback IP addresses from incoming messages on routers and other gateways. It is done in order to prevent loopback hosts from being repurposed by network attackers.

It is common for application software to use loopback features for local testing. If you send(LAN) messages to loopback addresses such as 127.0.0.1, none of the messages will reach outside the local area network. With this method, messages are delivered directly to the TCP/IP and receive queues without undergoing an Internet routing process. An additional destination port number is also included in the loopback message. By using these port numbers, applications can categorize text messages. Loopback addresses in IPv6 are represented by the following address: :1(0000:0000:0000:0000:0000:0000:0000:0001).

2. 0.0.0.0
In the past, it was used to indicate that there was no specific address to be directed to (a placeholder for ‘no specific address’) because the IP address is not routing. There are (IP) version 4 (IPv4) addresses ranging from 0.0.0.0 to 255.255.255.255. The IP address 0.0.0.0 has different meanings in different network environments. Addressing any device with this address, however, is not possible in general.

There is no route to the specified destination and the address is non-routable. If the message is viewed from a client or server device, its meaning may be different. Client and server devices are involved; the first is installed on the client device, while the second is embedded on the server device.

In the absence of an internet connection, the PC and other client devices usually display 0.0.0.0 as their IP address. Whenever a device is offline, it might be assigned this address by default. DHCP could also provide the address in case of failure. A device cannot be connected to any other device on the network using this address.

Devices can also be configured to use 0.0.0.0 as their subnet mask instead of their IP addresses. With this value, a subnet mask cannot be used. 0.0.0.0 is usually assigned to the IP address and network mask of a client computer.

0.0.0.0 is commonly used by firewall software and router software to block (or allow) all IP addresses.

Networking devices such as servers have a variety of network interfaces. TCP/IP software applications, which use 0.0.0.0 in their programming, analyze all network traffic on a multihomed device using all of the IP addresses assigned to the interfaces.

Now, let’s try to understand their differences:

127.0.0.1 IP Address	0.0.0.0 IP Address
It is a loopback address(localhost address).	It is a non-routable address.
This address is used to connect to the same machine or computer the end-user is using.	It indicates an invalid, unknown, or inapplicable end-user address(a ‘no particular address’ placeholder).
It is a special class A network address with the “loopback” function, meaning that a datagram sent from a higher-level protocol to the address will loop back. The network 127 addresses should never appear in any datagram ever sent to them.	It’s also a special address. Whenever an IP address in dotted-decimal notation is expected, it should parse as valid. When the parsed number is converted into a workable numeric form, the next step is determined by its value.
127.x.y.z also is another address of the computer. 127.0.0.0 is a loopback subnet and 127.255.255.255 is a broadcast address for the loopback subnet.	0.0.0.0 is not the address of anything.
Some servers, including MySQL, may only accept loopback connections.	For connection from outside networks or sources, the blind-address 0.0.0.0 is typically used.

A typical bind-address for connections from external networks or sources is 0.0.0.0. Some servers, including MySQL, accept connections only from loopback addresses, and may require an administrator to change the address to 0.0.0.0 in order to make external connections possible.