In classful addressing it is a network 3.0.0.0 with a netmask 255.0.0.0 (/8). In a classless, it is whatever has been advertised to me.
> Public BGP requires /24 or wider, and /24 cannot contain .0 or .255 as a valid IP, so the next widest is /23, which can contain even-numbered .x.255 and .x+1.0. If you're doing this on an internal network that permits narrower subnets, the narrowest available would be a /30, based (irregularly) on either .1.254 or .1.255:
It does not. You can advertise whatever the hell you want. What matters is what the other side would accept. It used to be that no one would accept anything less specific than /24 in a class C address space, /16 in a class B address space and /8 in a class A address space because the internet ran on AGS and maybe AGS+ which just did not have enough memory to do real classless. That stopped around the time GlobalCrossing showed up (or maybe around the time Mr V did terrible things while running AS7007). But anyway, ten years ago /27s are definitely accepted routes by those that comprised over 80% of the gloabal internet by the single origin prefixes.
Are there any publicly advertised, unsummarised /31s or /32s in the global BGP tables?
Only /31s could include .0 and .255 and those would end up on router point to point links.
> Are there any publicly advertised, unsummarised /31s or /32s in the global BGP tables?
I filter them out as martians, so I cannot answer this.
That requirement is about how you advertise your routes not how you subnetted the internal network. .0 and .255 of your advertised /24 can work fine as /32s on the server. Using the internet FW to NAT the network and broadcast addresses is another common trick to get more addresses, particularly when you only get an e.g. /29 from a carrier who is just routing that /29 to your specified next hop and 2 more addresses is a big bump.
Many home routers issue IPs using the follow netblock:
Network: 192.168.1.0 Netmask: 255.255.255.0 (/24) Broadcast: 192.168.1.255
Gateway: 192.168.1.1 (the first valid IP in that block) or less often .254 (the last valid IP in that block)
So for the past thirty years, nearly _every_ network block issued by _any_ network admin used /24, to the point that some things simply refuse .0 and .255 as valid IP addresses.
The suggestion upstream is to set up the IP 3.2.1.0, within _any_ network block that can contain it legally. If you adhere to strict netmasking, that'd be (this is from memory, correct me if I'm wrong):
Network: 3.2.0.0 Netmask: 255.255.254.0 (/23) Broadcast: 3.2.1.255
Which would provide a contiguous block of valid IPs from 3.2.0.1 through 3.2.1.254, _including_ the two perfectly valid IPs 3.2.0.255 and 3.2.1.0.
Those valid IPs end in .255 and .0, which is completely legal since they're in the middle of a /23, and the default gateway wouldn't be anywhere near them.
A lot of human-operated networks will blacklist issuing .0, .1, .254, .255 on the theory that it's simpler to just prohibit them and lose 4 IPs per /24 within a block, than deal with what's truly possible for a /23 or wider block.
EDIT:
Public BGP requires /24 or wider, and /24 cannot contain .0 or .255 as a valid IP, so the next widest is /23, which can contain even-numbered .x.255 and .x+1.0. If you're doing this on an internal network that permits narrower subnets, the narrowest available would be a /30, based (irregularly) on either .1.254 or .1.255:
Network: 3.2.0.254 (or .255) Netmask: 255.255.255.252 (/30) Broadcast: 3.2.1.1 (or .2)
With 3.2.1.0 as the live IP and either 3.2.0.255 or 3.2.1.1 as the in-network gateway for that device.