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Internet Protocol version 4 (IPv4) is the fourth version of the standard that routes Internet traffic and other packet-switched networks introduced in 1982 by the Internet Engineering Task Force (IETF). IPv4 is the most widely used version of the protocol despite the limitations of its 32-bit address space. With slightly less than 4.3 billion available unique addresses, the available number of addresses quickly began to run out. Without some clever ingenuity over the years that extended the life of the protocol, the pool of available addresses didn’t dry up until 2011.

What is an IP address?

An Internet Protocol address is a unique identifier for devices connected to a network. The unique identifier makes it possible for devices to find and communicate with each other. Initially, the main types of devices that required an IP address included network devices, such as computers, servers, routers, and printers. However, with the Internet of Things, the list includes cell phones, televisions, refrigerators, automobiles, light bulbs, or anything else capable of receiving or exchanging information over a network.

Understanding IPv4 addressing

An IPv4 address is a series of four eight-bit binary numbers separated by a decimal point. Although you may use any numbering system to represent a unique 32-bit number, most commonly you see IP addresses expressed in dot-decimal notation.

Site Dot-decimal Binary 10101100.11011001.10101000.11101110 00011111.00001101.01010100.00100100 10010111.01100101.00000000.01010100

Early IPv4 routing

Initially, the standard defined the first octet as a network identifier, but with only 256 unique values, the number of available networks quickly ran out. Several different changes made over the years have allowed for the extension of IPv4’s life. First came the division of the available addresses into five classes: A, B, C, D, and E.

The class system defined which class a network belongs in based on its first octet.

  • Class A network's first octet begins with 0. The first octet identifies the network. Class A supports 127 networks, each with 16 million hosts.
  • Class B network's first octet begins with 10. The first and second octets identify the network. Class B upports 16,000 networks, each with 65,000 hosts.
  • Class C network's first octet begins with 110. The first three octets identify the network. Class C supports 2 million networks, each with 254 hosts.
  • Class D network's first octet begins with 1110. Class D is reserved for multicast groups.
  • Class E network's first octet begins with 1111. Class E is Reserved for future use.

Each class used a different number of bits to identify the network affecting how many networks and hosts each class could accommodate. For example, Class C’s first three octets described the network, while the fourth described the host on the network.  Later the IETF replaced the class system, dubbed “classful,” with subnet masks that allowed for the dispersal of addresses on any address-bit boundary.

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IPv4 today

In 1993 the introduction of Classless Inter-Domain Routing (CIDR) gave greater flexibility for allocating blocks of addresses. CIDR adds a suffix to the IP address to identify how many of the leading bits represent the network address. For IPv4, that means a number between 0 and 32. The higher the suffix, the fewer available host address available on the network.

CIDR slowed the growth of routing tables and extended the life of IPv4 by reducing the number of wasted addresses that plagued the class system. CIDR is still the most widely used network routing method used today for both IPv4 and IPv6 routing.

IPv4 address exhaustion

2011 saw the last distribution of IPv4 address blocks to the five regional Internet registries, one of which ran out of addresses completely within the next few months. The individual Internet service providers keep IPv4 alive by recycling addresses as they become available.

As noted earlier, IPv4 has a cap just short of 4.3 billion available addresses. With the Internet's explosion in growth and the Internet of Things, the number of available addresses quickly depleted. To remedy the situation, the IETF released IPv6 with its 128-bit address space for a nearly inexhaustible 340 undecillions (340 followed by 37 zeros) available addresses. Learn more about IPv4 address exhaustion.

IPv4 and IPv6 compatibility

Although IPv4 and IPv6 use CIDR to handle their network and host addressing, the two protocols are not interchangeable. IPv6 also fixes many other networking issues inherent in IPv4, such as smaller routing tables, simplified packet headers, and uses multicast instead of broadcast.

A single device can support both IPv4 and IPv6. Duel-stack IP allows for a single router, switch, or server to process either address space. You cannot connect to an IPv6 only device using an IPv4 connection and vice-versa.

IPv4 speed

Some of the baggage IPv4 carries to extend the number of addresses does affect network speed. In the perfect IPv6 environment, IPv6 outperforms IPv4. However, the IPv6 network still requires work, and so depending on local architecture, IPv4 is often faster. An algorithm called Happy Eyeballs used by some browsers will test the speed of both network protocols and use the faster version.

DNS for IPv4 and IPv6

The Domain Name System (DNS) supports both protocols. The DNS stores the IP addresses for either or both and responds to each request for domain name resolution with both IP addresses (A site can have multiple addresses for either protocol).

DNS puts IPv4 addresses into the A record. The DNS system stores IPv6 addresses in the AAAA record. The client can then decide which protocol to use.

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Monitoring your IPv4 addresses

The IP address is a vulnerable part of the networking protocol. If a hacker gains access to the DNS settings, they can modify the IP addresses. In doing so, they can direct users to a malicious site, or just prevent users from accessing a destination. To protect against hacking, a DNS monitor can verify the IP address once per minute. A DNS monitor can also check and verify other records contained in your DNS, such as MX and NS records.

Monitor both IPv4 and IPv6

Just because both protocols route to the same server doesn’t mean that both protocols work. Explicitly monitoring IPv6 and IPv4 is possible with uptime monitoring (for websites and web services). Choose either protocol in the monitor settings and designate the monitoring checkpoints. For IPv6 designate checkpoints that only support IPv6 natively or use them all with IPv6 simulation over IPv4.

Which protocol does my site support?

Using the free DNS tool, enter a domain name, e.g., Click Start test.

The free DNS Lookup Tool resolves the address. That is, the tool queries the DNS system and retrieves your DNS records. Scroll down the results and look for the A and AAAA records. You may have multiple of either or none of the other.

  • If the results include an A record, the site supports IPv4 (most sites do).
  • If the results include an AAAA record, the site supports IPv6 (less common).
  • If the site has results for both, the site supports both.

Key points

  • IPv4 is the most widely used protocol over IPv6.
  • IPv6 fixes the address exhaustion problem with IPv4.
  • IPv4 uses a 32-bit addressing system.
  • IPv6 and IPv4 can exist on the same device with dual-stack enabled.
  • Happy Eyeballs is an algorithm that allows the device or browser to choose the faster protocol from a destination.
  • IPv6 will eventually replace IPv4, and IPv6 adoption grows by 5% each year.
  • A website or service while available on one protocol may error on the other. Monitor both IPv6 and IPv4 addresses for availability.

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