IPv4 vs IPv6: What’s the Difference?

IPv4 and IPv6 are the two versions of the Internet Protocol that assign addresses to devices on a network, where IPv4 uses 32-bit addresses and IPv6 uses 128-bit addresses. The Internet Engineering Task Force (IETF) defined IPv4 in RFC 791 in 1981 and IPv6 in RFC 8200. This article compares them across address length and format, the size of each address space, the IPv4 exhaustion problem and the role of NAT, the built-in features of IPv6, why the migration is still incomplete, and how the two protocols coexist.

What Is the Difference Between IPv4 and IPv6?

The difference between IPv4 and IPv6 is that IPv4 uses 32-bit addresses written as four decimal numbers, while IPv6 uses 128-bit addresses written as eight hexadecimal groups. The longer address is the root of every other difference, because it gives IPv6 a vastly larger pool and removes the need for the workarounds IPv4 depends on:

• Address space: IPv4 holds about 4.3 billion addresses; IPv6 holds about 340 undecillion, enough to end the shortage.
• Configuration: IPv4 hosts get an address manually or by DHCP; IPv6 hosts can self-assign one through stateless address autoconfiguration (SLAAC).
• NAT: IPv4 leans on Network Address Translation to share scarce addresses; IPv6 gives every device a globally unique address, so NAT is not required.
• Built-in features: IPv6 folds IPsec, a fixed header, and multicast into the base protocol, where IPv4 treats them as add-ons or omits them.

IPv4 and IPv6 both perform the same core task of addressing and routing packets; they differ in scale, configuration, and built-in features rather than in purpose. The general role of an address appears in the overview of an IP address, and the protocol that carries those addresses is covered in TCP/IP.

IPv4 vs IPv6 at a Glance

IPv4 is the established protocol that almost everything still speaks, while IPv6 is the long-term replacement built for a world with far more devices. The two are not rivals so much as old and new versions of the same idea:

What Is the Address Format of IPv4 and IPv6?

IPv4 writes an address as four decimal octets separated by dots, while IPv6 writes an address as eight groups of four hexadecimal digits separated by colons. The two formats reflect the different address lengths:

• IPv4 uses dotted decimal. An address such as 192.168.1.1 holds four octets, each from 0 to 255, for 32 bits total.
• IPv6 uses hexadecimal groups. An address such as 2001:0db8:0000:0000:0000:0000:0000:0001 holds eight groups of 16 bits, for 128 bits total.
• IPv6 allows compression. A run of consecutive zero groups collapses to a double colon, so the example above shortens to 2001:db8::1.
• Leading zeros drop in IPv6. A group such as 0db8 writes as db8, which shortens the address further.

How Large Is the Address Space of Each Protocol?

IPv4 provides about 4.3 billion addresses, and IPv6 provides about 340 undecillion addresses. The difference comes entirely from the address length.

IPv4 uses 32 bits, which gives 2 to the power of 32, or 4,294,967,296, total addresses. IPv6 uses 128 bits, which gives 2 to the power of 128, or roughly 340 trillion trillion trillion, addresses. The IPv6 space is large enough to assign a unique address to every device for the foreseeable future without reuse.

The figure of 340 undecillion writes as 3.4 followed by 38 digits. The host portion within a subnet is bounded by the subnet mask in IPv4 and by the prefix length in IPv6.

What Is IPv4 Address Exhaustion?

IPv4 address exhaustion is the depletion of the roughly 4.3 billion IPv4 addresses caused by the growth of internet-connected devices. The Internet Assigned Numbers Authority (IANA) allocated the final blocks of IPv4 addresses to the regional registries in February 2011.

The number of devices on the internet passed the IPv4 address count, which created a shortage. APNIC, the registry for the Asia-Pacific region, was the first to reach its last block in April 2011, and the American Registry for Internet Numbers (ARIN) exhausted its free IPv4 pool in September 2015. The shortage drove two responses.

The first response was Network Address Translation, which lets many devices share one public address. The second was IPv6, which removes the shortage through its far larger address space. The exhaustion of IPv4 addresses is the primary reason the internet moves toward IPv6.

How Does NAT Extend IPv4?

Network Address Translation lets many devices on a private network share a single public IPv4 address, which delays IPv4 exhaustion. A router performs the translation between private and public addresses.

A home network assigns private IPv4 addresses such as 192.168.1.20 to each device. The router translates those private addresses to one public address when traffic leaves for the internet, and translates return traffic back to the correct private device. NAT conserves public addresses, because one public address serves an entire network.

NAT also has the side effect of hiding internal addresses from the public internet. The full mechanism appears in the overview of Network Address Translation. IPv6 removes the need for NAT, because every device receives a globally unique address.

Why Is the Migration to IPv6 Taking So Long?

The migration is slow because NAT removed the urgency, the two protocols are not backward compatible, and IPv6 brings no visible day-to-day benefit to most users. Several forces pull in the same direction:

• NAT relieved the pressure. Address sharing made IPv4 scarcity manageable, so the cost of migrating outweighed the perceived benefit.
• No immediate payoff. By design IPv6 is “IPv4 with bigger addresses,” not faster or safer out of the box, which weakens the case for end users and many businesses.
• Not backward compatible. An IPv6-only host cannot talk to an IPv4-only host directly, so organizations must run both or add translation, increasing complexity.
• A working secondary market. Companies can buy or lease IPv4 addresses, which is often seen as simpler in the short term than a full IPv6 rollout.

What Features Does IPv6 Add?

IPv6 adds autoconfiguration, built-in IPsec, a simplified header, and the removal of NAT as standard features. The protocol was designed to correct limitations of IPv4:

• Stateless address autoconfiguration assigns addresses. SLAAC lets a device generate its own IPv6 address from the network prefix without a DHCP server.
• IPsec is part of the protocol. IPv6 includes authentication and encryption support in the base specification, where IPv4 treats IPsec as an add-on.
• A fixed header simplifies routing. The IPv6 header is a fixed 40 bytes, which lets routers process packets with less overhead than the variable IPv4 header.
• Multicast replaces broadcast. IPv6 removes broadcast and uses multicast and anycast, which reduces unnecessary traffic on the network.

How Far Has IPv6 Adoption Progressed?

IPv6 reached a majority of Google traffic for the first time in March 2026, while most networks still run IPv4 and IPv6 together through dual-stack. Adoption varies widely by country.

According to Google IPv6 statistics, native IPv6 access crossed 50 percent of its users on 28 March 2026, a milestone that took roughly eighteen years to reach. APNIC, which measures capability a different way, records about 42 percent worldwide; the two figures bracket the likely real range. Some countries report IPv6 capability well above 60 percent, while others remain below 10 percent.

Dual-stack operation runs IPv4 and IPv6 on the same device at the same time, so the device uses whichever protocol a destination supports. This allows a gradual transition, because a network does not switch off IPv4 until every destination it needs supports IPv6, and many legacy systems still speak only IPv4.

How Does Security Differ Between IPv4 and IPv6?

IPv6 includes IPsec support in its base specification, while IPv4 added IPsec later as an optional extension. The security difference covers encryption, address scanning, and privacy.

• IPsec authenticates and encrypts traffic. IPv6 defines IPsec headers in the core protocol, which standardizes end-to-end authentication and encryption.
• The larger IPv6 space resists scanning. A subnet with 2 to the power of 64 addresses is too large to scan address by address, unlike a small IPv4 subnet.
• Privacy extensions hide the interface identifier. RFC 4941 lets a device rotate the host portion of its IPv6 address so a server cannot track it over time.
• The loss of NAT removes one barrier. A globally routable IPv6 address exposes a device directly, so a firewall replaces the incidental filtering that NAT provided.

IPv6 does not make a network secure on its own, because misconfiguration affects both protocols. A firewall remains necessary on IPv6, since every device holds a public address rather than a private one behind NAT.

Are IPv4 and IPv6 Compatible?

IPv4 and IPv6 are not directly compatible, because the two protocols use different address lengths and packet formats. A pure IPv6 device cannot communicate with a pure IPv4 device without a translation mechanism:

• Dual-stack runs both protocols. A device with both IPv4 and IPv6 enabled communicates with hosts on either protocol.
• Tunneling carries IPv6 over IPv4. A tunnel wraps IPv6 packets inside IPv4 packets to cross an IPv4-only network segment.
• Translation converts between protocols. NAT64 and similar mechanisms translate IPv6 traffic to IPv4 so an IPv6-only client reaches an IPv4-only server.
• Applications may need updates. Software that stores addresses in a 32-bit field requires changes to handle the longer IPv6 address.

What Are the Types of IPv6 Addresses?

IPv6 defines three address types, unicast, multicast, and anycast, and removes the broadcast type that IPv4 uses. Each type controls how many interfaces a packet reaches:

• A unicast address identifies one interface. A packet sent to a unicast address reaches a single device, the same role the common IPv4 address fills.
• A multicast address reaches a group. A packet sent to a multicast address, which begins with ff00::/8, reaches every interface that joined the group.
• An anycast address reaches the nearest member. A packet sent to an anycast address reaches the closest of several interfaces that share the address.
• A link-local address stays on one link. An address in the fe80::/10 range works only on the local segment and is generated automatically for every IPv6 interface.

The removal of broadcast reduces unnecessary traffic, because multicast delivers only to interested devices. Every IPv6 interface holds a link-local address in addition to any global address.

How Does IPv6 Support the Internet of Things?

IPv6 supports the Internet of Things by providing enough addresses to assign a unique global address to every connected device without NAT. The sheer number of connected devices is the reason the address space matters.

The International Data Corporation estimated tens of billions of connected devices by the middle of the 2020s, a figure that exceeds the entire IPv4 address space. IPv6 assigns each sensor, meter, and appliance its own routable address, which removes the address-sharing workaround that NAT requires. A direct global address simplifies device management and remote access, because each device is reachable without port forwarding.

Stateless address autoconfiguration also lets a device join a network and obtain an address without a DHCP server, which suits large fleets of low-power devices. The address space of IPv6 is the feature that makes this scale possible.

Last Thoughts on IPv4 vs IPv6

IPv4 and IPv6 are the two Internet Protocol versions, where IPv4 uses 32-bit dotted-decimal addresses and IPv6 uses 128-bit hexadecimal addresses. The 4.3 billion IPv4 addresses ran short as internet devices multiplied, which led to Network Address Translation and the move to the roughly 340 undecillion addresses of IPv6. The migration is slow because NAT removed the urgency and the two protocols are not backward compatible, so networks run both through dual-stack rather than switching over at once.

The trend is clear even if the timeline is long: IPv6 crossed half of Google traffic in 2026 and continues to grow. The address shortage that NAT addresses is covered in the overview of Network Address Translation, and these protocols sit within the broader set of topics on the computer network hub.

Key Takeaways:

• IPv4 uses 32-bit addresses (about 4.3 billion) and IPv6 uses 128-bit addresses (about 340 undecillion).
• IPv4 addresses ran out, with IANA allocating the last blocks in 2011, which is why IPv6 exists.
• NAT lets many devices share one public IPv4 address, which extended IPv4 and slowed the move to IPv6.
• IPv6 builds in autoconfiguration, IPsec, a fixed header, and multicast, and needs no NAT.
• The two protocols are not backward compatible, so networks run dual-stack during the transition.
• Google measured IPv6 crossing 50 percent of its traffic in March 2026, while APNIC measures about 42 percent worldwide.

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