IPv6 subnetting basic rules?

[notaciscoguy]

hello all,

I have a quick question about subnetting IPv6. in IPv4, when you subnet, you are not allowed to use certain addresses in the subnet, namely first and last address, because they are network and broadcast. however, I understand that there is no IPv6 broadcast anymore, so does this mean that you are now allowed to assign the last address in a block with IPv6, or has it been reserved for something else? also I am assuming the first address is still unusable because it is technically the network address. is this assumption correct? 

[Jose Quesada]

Hi,
Here I share a link that could help you understanding this issue: https://community.infoblox.com/t5/IPv6-CoE-Blog/When-is-an-IPv6-address-an-address-and-when-is-it-a-prefix/ba-p/6901
Broadcast and all-zero IPv6 addresses are legit and can be used for hosts addressing, but, best practices do not recommend using them, to avoid confusion on humans reading those addresses, but network devices will accept them with no issues.
Quotes from the link I shared: "IPv6 is a bit more confusing because we don't have the concept of a network and broadcast address (top and bottom addresses in IPv4 for a given subnet that are held in reserve to "describe" the network and the all hosts)"

"...avoid "all-zero" addresses so that it is easier to tell the difference between a prefix and an address. This is an operations principal to help reduce errors and mistakes."

I hope this helps.

[Harold Ritter]

Some addresses are also reserved for anycast purposes as described by RFC4291, section 2.6.1 and  RFC2526.

IOS will issue the following warning if you configure any of those addresses.

CE1(config-if)#ipv6 address 2001:db8:12ff:ffff::/64
%Loopback0: Warning: 2001:DB8:12FF:FFFF::/64 is a Subnet Router Anycast
CE1(config-if)#ipv6 address 2001:db8:12ff:1234:ffff:ffff:ffff:ffff/64
%Loopback0: Warning: 2001:DB8:12FF:1234:FFFF:FFFF:FFFF:FFFF/64 is a Reserved Anycast

Regards,