On 2023-07-25, Zack Newman <z...@philomathiclife.com> wrote: > On 7/25/23 06:03, Stuart Henderson wrote: >> 217.169.18.56 is a network address (mask it out against the netmask, >> the remaining "host bits" are all zeroes), you cannot use this (or the >> broadcast address) as a host address > > I am sure you were not trying to be "technical"; but for people that > don't already possess the knowledge, I just wanted to say that there > are two exceptions to the rule that you shouldn't assign the network or > broadcast address to a host: /31 (https://www.rfc-editor.org/rfc/rfc3021) > and /32 networks. I also have a /29 network of globally routable IPv4 > addresses, and I use all 8 of those precious IPs by carving out 8 /32s > and assigning each one to a host.
true, though Nx/32 won't let you route it to other hosts without a lot of hassle, and /31 may work or may not work depending on the other host's IP stack and other software in use (some things don't expect /31), and it doesn't actually give you any more available addresses unless you only have a routed /30. you can also put the all-0/all-1 addresses as /32 aliases and use them for NAT (i.e. nat-to, rdr-to, or binat-to, to a machine on an RFC1918-numbered network) while using the rest as normal on another nic. or there's a bodge if you don't need to talk to people on the surrounding addresses, where you set the netmask wider than you really have, say you have 192.0.2.80/28 from the ISP $ ipcalc 192.0.2.80/28 address : 192.0.2.80 netmask : 255.255.255.240 (0xfffffff0) network : 192.0.2.80 /28 broadcast : 192.0.2.95 host min : 192.0.2.81 host max : 192.0.2.94 hosts/net : 14 you could configure as /26 $ ipcalc 192.0.2.80/26 address : 192.0.2.80 netmask : 255.255.255.192 (0xffffffc0) network : 192.0.2.64 /26 broadcast : 192.0.2.127 host min : 192.0.2.65 host max : 192.0.2.126 hosts/net : 62 and actually use the 16 addresses .80-.95 (depending on how the routed subnet is aligned you may need to go to a wider prefix length, maybe as much as /23 if it starts on .0 or ends on .255). but none of these are things I think the OP should be concerned with at this time hence not suggesting them. :)
