Add an IPv6 address defaults to 0th address

[ktims]

Issue type

[ ] Feature request
[x] Bug report
[ ] Documentation

Environment

• Python version: 3.5.3
• NetBox version: 2.2.2

Description

When adding an IPv6 address to an existing prefix, the 'all-zeroes' host address seems to be considered available, and is filled in the input box, even if other IPs in the subnet are allocated, so the default is never sensible.

The all-zeroes address is reserved by RFC4291 2.6.1 as the subnet-router anycast address. Its manual assignment only makes sense on /127s, where RFC6164 asserts that the subnet-anycast mechanism MUST be disabled. At the very least, it isn't a sensible default, as operationally it should never be used for unicast, and many devices will block its usage.

Furthermore, the ipam/prefixes/<id>/available-ips endpoint appears to sort of 'do the right thing' and consider the 1st address to be the first available rather than the 0th. However, on /127s, it asserts that 0 IPs are available.

So:

• When adding an IPv6 address, the default form-filled value should be the first available IP that is not all-zeros in the host portion, except when the mask length is /127
• The ipam/prefixes/<id>/available-ips should be consistent with the /127 exception

[jeremystretch]

The same thing happens with IPv4. NetBox is just prepopulating the network. It's not automatically choosing the first available IP because IMO it's not fair to assume that's what the user wants to add.

[ktims]

I see... this doesn't seem like very useful behaviour, as you're never going to want to assign the network address to something. But I don't really use the 'Add IP Address' button.

Where I actually noticed the discrepancy was on the IP Addresses tab, which changes the flavour of this bug slightly. In IPv6, the 0th address seems to be considered 'available' even when it's not a pool. It adds a '1 IP available' row, and is the default when clicking 'Many IPs available' or any other free block that starts at the bottom of the subnet.

[jeremystretch]

Guess we'll just default to the first available IP and hope people are paying attention.