The Difference Between IPV6 vs IPV4

Don’t be afraid to use IPv6, It’s not a whole lot different from IPv4. Let’s look at the IPv6 specification here,

IPv6 Basics

Taking a first look at IPv6 can be overwhelming, but in reality, the addressing scheme is exactly the same as IPv4. For example, it would be possible to write an IPv4 address as FFFF:FFFF, which would equate to Conversely, we could write an IPv6 address as, which would be FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF.

That’s 128-bits of an address space versus the 32-bits in IPv4. This equates to an unprecedented amount of addresses. It would mean over 250 addresses for every observable star in the known universe. So it’s going to take a while to use all of them unless we waste it all needlessly. I can’t envision us using all this address space until we populate other planets, even if we gave every grain of sand in the world an IP address.

So why is IPv6 in hexadecimal format? A quick search shows a few different answers, but to me, it’s easier to concatenate and easier to read than 16 numerical digits. For example, if a large amount of the address space is zeroes it can be concatenated 1234::5678:1. Granted this can only be done one time, 1234::1::45 is invalid. Just like in IPv4 leading zeros can be omitted, however, I find it easier to write it all out:

2001:1850:1:0:104::8a is also 2001:1850:0001:0000:0104:0000:0000:008a

Which also can be 2001:1850:1:0:104:0:0:8a

This looks confusing, so writing it all out (putting ALL the zeros) when taking notes or preparing policies will help understand it better.

IPv6 is another address family and another protocol, meaning it is a completely separate set of routing and adjacency tables and even its own ethernet frame type. This means in normal terms that IPv6 is totally independent of IPv4 and doesn’t even know IPv4 exists. On an existing IPv4 network, a new IPv6 network will be created on every device as if setting up a completely new network installation. Think about this for a server, an IPv4 default gateway will NOT work for IPv6 even though it might be the same MAC address, the IPv6 has to be set separately.

Other than the addressing scheme and hexadecimal, IPv6 is exactly the same as IPv4 for subnetting and routing purposes. A subnet is still a subnet, a /24 in IPv4 is simply a /120 in IPv6, the same amount of IP addresses. Under the hood for routing, IPv6 does have some technical changes which increase routing performance such as a much simpler header format.

But wait! You read about IPv6 and it says the smallest subnet it supposed to be a /64? This is true but not true, just like the smallest subnet in IPv4 was a “Class C” before it went classless (CIDR). However there is a reason, and an RFC to back it up why a /64 was selected. Certain features of IPv6 require a /64 at the moment and may not in the future.

Question and Answer Omnibus

So, no NAT in IPv6?

Well, while it’s entirely possible to DO address translation, there’s no need for it due to the number of addresses available. A stateful firewall is all that is needed.

How about neighbor resolution?

In IPv4 we know this as ARP. Here is a fundamental difference in the way IPv6 works vs IPv4 underneath it all. While this makes no functional difference in how the protocol functions (ex. With TCP or UDP or ICMP, etc) it does change how it forms an adjacency/neighbor. ARP does not exist in IPv6, instead, it’s called neighbor discovery and it uses ICMP. Many of us are probably used to filtering ICMP by now and since it plays an important role in IPv6 discovery neighbors as well as the actual operation of the IPv6 protocol itself.

For example, fragmentation is ONLY performed by endpoints in IPv6 (the hosts talking to each other), and not by any router in between. ICMP is used to determine if packets need to be fragmented or not. This is an ICMP “Type 2” IPv6 packet. Neighbor discovery is entirely done with ICMP via multicast and unicast.

IPv6 does not use broadcasts! There is no ‘broadcast’ IP or ‘network’ IP address in an IPv6 subnet. The last IP is usable, unlike in IPv4.

Link local IPs?

Wait we saw these 169.254.x.x IPs in IPv4 but only in extremely rare instances were they ever used. How are they used in IPv6? This is a major difference from IPv4! This is also an annoying difference. It does change how things operate and what filters need to be put in place. Link local IPs are in the FE80::/10 range, and are, unless otherwise specified, automatically configured by the devices on their interfaces. This IP range is specified as unroutable on all routing equipment and should not be forwarded, thus the name link local or LAN only. This means that every IPv6 interface will have a least two IP addresses configured on it for connectivity outside of the LAN. You may have noticed a link local IP on a server that IPv6 is enabled on but no IP address has been configured yet. This is normal.


IPv6 QoS works exactly as IPv4, with the exception that IPv6 has a new flow label field added into the header to help with marking flows and traffic class designation. Since this is widely unused at the moment it isn’t worth discussing here, but noted anyway as a difference.

Security in IPv6?

No real difference here from IPv4, although it has built-in support for IPSEC, that can’t be counted on in all circumstances (ex. neighbor discovery still uses ICMP, ICMP messages still need to be sent to hosts unencrypted). IPSEC also is available for IPv4. IPv6 neighbor discovery to some is less secure than ARP. While it is a lot more complicated to filter, the security differences are negligible.

What does this mean for system administrators and firewall managers?

For IPv6 on a server, the main difference is the neighbor resolution. Certain ICMP types (133-137) need to be allowed in the firewall which allow neighbor resolution to work. FE80::/10 should be allowed for these ICMP messages also. You cannot simply filter everything except the destination IPv6 IP to the server, the link local must be allowed as well.

If you are wondering why IPv6 seems broken when you add it to a server, check the firewall.

Firewall admins should allow type 1-4 (err msgs) and 128-129 (echoes) at the least to allow proper operation and ping testing.

On the next blog, we will talk about DHCPv6, DHCP-PD, Mobility and privacy extensions, IPV6 header breakdown, Multicasting, Neighbor Discovery in depth, SLAAC, SEND, IPSEC.

TL;DR Differences Between IPv6 vs IPv4:

  • 128-bit address versus 32-bit address
  • Different Ethernet frame type (0x86DD) [IPv4 is 0x0800]
  • No broadcast or network address
  • Hex instead of decimal notation
  • No ARP in IPv6 uses ICMPv6 neighbor solicitation with multicast
  • uses link-local IP addresses (which are UNROUTABLE) autoassigned by hardware id (derived from MAC address) to communicate neighbor discovery, autoconfiguration
  • Built-in multicasting
  • Ipv6 does not require IPv4 to operate nor does it interfere with IPv4 operation, and should be treated as such: meaning: on servers, IPv6 will have its own address, gateway, mask, etc.
  • You cannot NAT directly IPv4 to IPv6 or IPv6 to IPv4 although it can be proxied **
  • For DNS, IPv6 is AAAA instead of A and reverse is (see DNS section below)
  • Jumbo JUMBO JUMBO datagrams, did I mention Jumbo? 32 bit number for window size (4 gig!)
  • ICMP replies from routers for MTU error responses
  • Header checksum is removed from the top IP level (deemed unnecessary, but I disagree)
  • Mobility and privacy extensions
  • DHCPv6 with DHCP-PD (prefix delegation)


** There are some options to NAT (port translation) and NAT64 between IPv4/IPv6 but it isn’t a direct 1 to 1 mapping

Wikipedia also has a wonderful page on IPv6,