Are IP addresses with a 0 in the last octet valid?

In my case, I have the the following netmask

What about a 0 for the other octets?

  • 14
    others have answered, but we run /23s in our DHCP ranges, meaning that the middle .255 and .0 addresses of the two /24s get assigned to clients. Works fine. Sometimes "knowledgeable" users freak out a little thinking they've pulled an invalid IP, but from a networking POV it works fine.
    – jj33
    May 21, 2009 at 21:26
  • 1
    See Also: What is the network address X.Y.Z.0 used for?
    – voretaq7
    Nov 19, 2012 at 23:07

6 Answers 6


It depends on the subnet of the IP address in question. In general, the first and last addresses in a subnet are used as the network identifier and broadcast address, respectively. All other addresses in the subnet can be assigned to hosts on that subnet.

For example, IP addresses of networks with subnet masks of at least 24 bits ending in .0 or .255 can never be assigned to hosts. Such "last" addresses of a subnet are considered "broadcast" addresses and all hosts on the corresponding subnet will respond to it.

Theoretically, there could be situations where you can assign an address ending in .0: for example, if you have a subnet like, you are allowed to assign a host the address It could create confusion though, so it's not a very common practice.

In your example with subnet (22 bit subnet mask)

means subnet ID, a host address range from to and a broadcast address So in theory, your example would be allowed as a valid host address.

  • 5
    One addition. In the past I have had to deal with some older software that had problems with using a .0 address in places where it was a perfectly legal thing to do.
    – Zoredache
    May 21, 2009 at 22:56
  • And no answer to this question would be complete without a reference to the CIDR RFCs: RFC1518 and RFC1519 which define all this.
    – pjz
    May 22, 2009 at 2:42
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    RFC 1519 is obsolete for a long time. The current version is RFC 4632.
    – bortzmeyer
    May 22, 2009 at 6:11
  • 3
    Just got assigned a dot zero by an Amazon EC2 instance. They are sure maximising the IP's they have.
    – hookenz
    Jun 5, 2017 at 20:38
  • @bortzmeyer, RFC 4632 is simply a BEST CURRENT PRACTICE, while RFC 1519 is a standards track RFC.
    – Ron Maupin
    May 27, 2019 at 3:39

answer to your question depends on the netmask. in general statement 'IP addresses ending in .0 or .255 is invalid' is false. take - it's valid ip address.

also aka is valid.

that was the theory. most reasonable network devices [ including linux servers, windows boxes, cisco / hp / etc ] will work fine with such address, but i've seen dlink and other low-end network appliance [ routers, access points ] not accepting such addresses.


I found this, which claims that it is valid, depending on your subnet mask.



I'd like to add a bit about 0 for the other octets:

This one is easy: it's no problem at all, as the fairly common private network address shows.

Of course an even more obvious example would be

  • The question isn't asking about zeroes in the other octets.
    – slang
    Aug 31, 2017 at 16:27
  • 6
    @slang: Except it literally asks about exactly that in the last sentence. Aug 31, 2017 at 16:28

I have run into problems with remote networks denying IP addresses from my network if they ended with 0 (or 255) and they were from the class C range, since anything ending with 0 would be an invalid class C network.

This was a few years ago; I don't know if anyone still blocks addresses like that or not.

  • That just sounds like your firewall/software is a bit daft ;)
    – nixgeek
    Jul 4, 2009 at 15:51
  • Every IP address on my network except .0 or .255 could access every site, IP addresses ending in .0 and .255 could access 95% of sites, but there were two or three completely different sites that they couldn't access. If it was my firewall/software, I sure couldn't figure out how. Jul 4, 2009 at 17:22
  • 1
    These must be using firewalls configured by the same kind of people who block all ICMP and end up breaking PMTUD, or block all "invalid" TCP flags and end up breaking ECN.
    – CesarB
    Jul 5, 2009 at 0:11
  • Microsoft servers allegedly do it even today. No Windows Update for you. But Microsoft has been known to break the rules since forever.
    – Zdenek
    Feb 20, 2016 at 12:15

Just something I found that's probably noteworthy:

If you're running R-fx networks' APF script for iptables, it drops all traffic to

We had a BT customer with an address ending in .255 with a prefix of /21 .. Technically a valid IP address, however the guys at R-fx networks think there is cause for dropping packets for these addresses.

  • they are choosing to drop packets to most likely for security purposes. 1) DOS attacks can happen by leveraging the power of a broadcast packet and 2) to completely privatize the network so no hosts can broadcast. see en.wikipedia.org/wiki/Broadcast_traffic#Security
    – zamnuts
    Jul 3, 2013 at 23:41

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