Computers & Hardware > Networking

Dot zero / zeroth device in subnet

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Weaver:
What problems if any am I likely to encounter if I allocate the first IP v4 address in a subset to a device?  That is: x.x.x.0 or dot base-plus-zero if the subnet is smaller than a /24 and doesn't start on .0. My current main IPv4 /26 is based at .192 and out of old-fashioned superstition I have not allocated .192 to any box. I have never actually been given a real concrete reason not to do it, just told it bad juju.

Same question with IPv6 too. I used xxxx:1 rather than :0 for a node a while back for example.

The obvious answer is to try it out and pray.

(I get the feeling I have asked this question many years earlier, if so: mea culpa maxima.)

andyfitter:
I don’t think you can ever user the first or last address in a subnet for an individual device in IPv4. The first address is the subnet identifier used for routing and the last is the broadcast address.

Weaver:
Agree Andy. The last address is obvious.

However I've never been able to find out why one can't use the first address. Maybe need to express it in these terms:
    1. Exactly what bad thing is going to happen to you if you do?
or
    2. Exactly what prevents you from just doing it?

I only just realised that the things I have read are ambiguous and maybe some people have been talking about (2), the _ability_ to do so. Epistemic modality vs deontic modality.

I am able to commit the crime, but am I able to get away with it? Or would others not be able to get away with it, or even not be able to commit it?

d2d4j:
Hi weaver

Sorry if I’m not understanding

Do you mean an internal ip range or ipv4 range externally assigned to you

If internal, I don’t think the nic would let you assign .0 but if it did, you would not be able to use it I don’t think, and/or it could cause routing issues

If external ip range, same thing but provider may also not be happy with you

Many thanks

John

burakkucat:
Let us consider the 16-bit private address space, 192.168.X.X/16

If I was performing an nmap scan, I could use 192.168.0.0/16 or 192.168.[0-255].0/24 as the address target range. (In the latter case, I have used [0-255] just to signify that the number in that position is any one from the range so specified and not something funky for nmap to perform.) Explicitly: 192.168.0.0/24 specifies the range 192.168.0.0 to 192.168.0.255 and 192.168.0.0/16 specifies the range 192.168.0.0 to 192.168.255.255 (if I've typed it correctly).

Now I perform two simple ping tests --

[Duo2 ~]$ ping -c 1 192.168.1.0
Do you want to ping broadcast? Then -b. If not, check your local firewall rules.
[Duo2 ~]$ ping -c 1 192.168.1.255
Do you want to ping broadcast? Then -b. If not, check your local firewall rules.
[Duo2 ~]$

In both cases the ping command responds as if both target addresses are broadcast addresses. Possibly a bug in the ping command.

What use or relevance is the above?  :shrug2:

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