How many public useable IPv6 ips in a /64 range?

Tom John

2/8/24, 8:22 PM

so I know that a /64 IPv6 range has 18,446,744,073,709,551,616 addresses, but I was told a majority of these IPs can only be use for local ips on LAN.

How many useable public ipv6 ip addresses are there in a /64? Thank you.

707

2 + 1

ipv6

vidarlo

2/8/24, 8:40 PM

*I was told* is a really really bad source. What makes you believe it? Did they give any reasons? If not, go back and ask why.

Score:1

Server

TomTom

2/8/24, 8:34 PM

That can not be answered like this because it does not depend on the /64.

An IP Address is 128 bit. 64 of that are the IP address within a network, the OTHER 64 bit are the network.

And the networks are separated - private address space i.e. LAN only) is NOT defined by a specific IP address within a /64 but by the NETWORK being in a specific range.

https://en.wikipedia.org/wiki/IPv6_address

has an explanation of all the prefixes and links to their definition As yo ucan see there, i.e., LINK LOCAL addresses (which are not even LAN - they are specific to a hardware link) have 64 bit addresses at the end (which makes sense - this was once supposed to default to the MAC Address for Internet) but the PREFIX is different.. They start (left, bits) with 1111111010 - OUTSIDE the 64 bit of what you call the ip address.

So, the usable IP Addresses in a Range are either nearly all (I think the usual all0 and all 1 are excluded) or... ZERO, depending on the OTHER 64 bit in the address, before the /64.

+ 1

Tom John

2/8/24, 8:41 PM

Thank you TomTom - Tom John

Score:0

Server

John Mahowald

2/10/24, 1:44 AM

More IP address than any subnet can possibly use.

The number of hosts per net, for address planning purposes, does not matter on IPv6. What actually matters is number of /64 subnets.

Say the local site has 2001:db8:835a::/48 and a server net is 2001:db8:835a:b959::/64 out of that. This subnet is the atom of address planning. Lower 64 bits make up the interface half of the IP address, and can be anything. Examples: 2001:db8:835a:b959::1 if you want to omit a lot of zeros, 2001:db8:835a:b959:dd19:6250:b4e:fea6 for using randomly auto assigned. Assuming properly routed subnets, the entire /64 is available to use. Less that you care about how many there are, but more you have 64 bits to play with for address generation.

When you need another VLAN or security zone or whatever, get a different /64 by changing b959.

When the prefix is allocated from global unicast space, the addresses are "public" and can participate in end to end over the internet. As always, this can be prevented by firewalls. Say host 2001:db8:835a:b959::1 has a default router fe80::44f7:8bff:fe46:7816 and is allowed internet access. That 1 address is the source address of the packet. No NAT required.

Those host addresses also work fine as for LAN communications. In much the same way, except the specific destination network is known, so it doesn't go out an internet interface.

+ 0

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