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I know that if I have a network 83.23.159.0/24 then I have 254 usable host IP addresses because:

83.23.159.0      (in binary: host portion all zeros) is the subnet address
83.23.159.1-254  are host addresses
83.23.159.255    (in binary: host portion all ones) is the broadcast address

I understand the use for a broadcast address, but I don't understand what the subnet address is ever used for. I can't see any reason that an IP packet's destination address would be set to the subnet address, so why does the subnet itself need an address if it is never going to be the endpoint for AN IP flow? To me it seems like a waste to not allow this address to be used as a host address.

To summarise, my questions are:

  1. Is an IP packet's destination ever set to the subnet IP address?
  2. If yes, in what cases and why?
  3. If no, then why not free up that address for any host to use?
share|improve this question
1  
ifconfig doesn't complain when you set a 24-net's host address to 0 or 255. Just try using it. –  ott-- Nov 22 '12 at 18:22
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I did. It seems to work for a .0. But it might break older software. .255 will also work if you use not use any protocols which require broadcast. I guess you can set it up if you manually configure the ARP tables with permanent addresses on all computers, but yuk! –  Hennes Nov 22 '12 at 18:59
    
possible duplicate of How does IPv4 Subnetting Work? –  John Gardeniers Nov 25 '12 at 2:26
    
I disagree with that John. I rechecked it and it does not address the specific question asked. Which is rather an edge case. –  Hennes Nov 25 '12 at 14:52
    
The first and last IP address in a subnet are reserved. The first is used as the network id the last is used for broadcast. depending on the mask, sometimes the first is 0 and the last is 255. –  Ben Plont Dec 10 at 19:04

6 Answers 6

up vote 7 down vote accepted

Is an IP packet's destination ever set to the subnet IP address?

Yes. It is a valid IP so it can be used.

If yes, in what cases and why?

It is simply one of the 255 usable IPs in a /24

If no, then why not free up that address for any host to use?

If you have ancient hardware then you need to check if it uses the first or the last address as network address. (.0 or .255 for networks with mask FF.FF.FF.00)

This makes it a good habit to skip that IP. And habits learned long ago are hard to ignore.

And people who do not know the background do not use it 'because other do not use it either, so it must be wrong to use it' or because they do not realise that '0' can be a first number.

[Edit] Grezzo just tested it on Windows XP where windows network GUI 'helpfully' prevented this setting. Windows 7 has the same behaviour. I then tried it on a non-windows host where it just works. If you use windows then you might have to configure your network manually via IPconfig to set it to all zeros.

192.168.1.0_on_win7 192.168.1.0_on_FreeBSD

[Edit 2]

The longer I work with this the more confused I get.

Rfc4632 - Classless Inter-domain Routing does not seem to forbid it, but neither allows it explicitly.

This ServerFault post mentions: "For historical reasons many OSes treat the first address as a broadcast. For example, pinging x.x.x.0 from OS X, Linux, and Solaris on my local (/24) network gets responses. Windows doesn't let you ping the first address by default but you might be able to enable it using the SetIPUseZeroBroadcast WMI method. I wonder if you could get away with using .0 as a host address on an all-Windows network.".

It is the same question, but not an answer.

The network address is also used in routing tables. But I fail to see why it would not work due to that. The same notation in the routing tables would route to the right network. Once on the right network it would arrive at a PC with IP 0.

(All of this for a 192.168.1/24.
If you used 192.168.0/23 then 192.168.1.0 would be a valid and safe value in the middle of the range)

[Edit 3]

One more link to the same question. It seems somewhat popular on stack exchange:

http://superuser.com/questions/379451/why-can-a-network-address-not-be-a-valid-host-address

And one thought:

Destination_IP is probably *AND*ed with the network mast (a fast operation in hardware) before being compared to the entries in the routing tables. But:

(A semi-random IP) 192.168.0.42 AND 255.255.255.0 would yield 192.168.0.0
But 192.168.0.0 AND 255.255.255.0 would also yield 192.168.0.0

share|improve this answer
    
So to clarify, are you saying that x.x.x.0/24 is a valid IP address for a host? If that is so, then why (when I try to set my IP address to 192.168.1.0 with a netmask of 255.255.255.0) does windows XP say "The combination of IP address and subnet mask is invalid. All of the bits in the host address portion of the IP address are set to 0. Please enter a valid combination of IP address and subnet mask." –  Grezzo Nov 22 '12 at 17:24
3  
Windows is wrong. (Or said with less force: windows is overly careful to work with any combination of systems). I just tried this on my windows 7 system and I got the same error as you did. I installed a clean FreeBSD system and tested it where it just works. (I'll add that to the post) –  Hennes Nov 22 '12 at 17:45
    
Thanks, you've been really helpful. Just wanted to add that the GUI in OS X won't let you do it either, but I bet ifconfig would. I can't understand why m0ntassar's answer has got more up votes - it doesn't even try to answer my question. One more thing; when you say "If you have ancient hardware then you need to check if it uses the first or the last address as network address." do you mean broadcast address. –  Grezzo Nov 22 '12 at 19:01
    
Yes, I meant broadcast address. Damn, where is that "edit comment older than 5 minutes" button. –  Hennes Nov 22 '12 at 19:03
    
Sweet, I just tried pinging .0 and .255, both times my PS3 replied (everything else is wireless so I guess that's why it got in there first) from .65 so that confirms that in my network both are used for broadcast which is why (normally) we shouldn't use either for a real host address. –  Grezzo Nov 22 '12 at 19:08

RFC 1122 ("Requirements for Internet Hosts -- Communication Layers") prohibits it:

IP addresses are not permitted to have the value 0 or -1 for any of the < Host-number>, < Network-number>, or < Subnet- number> fields

share|improve this answer
1  
True as far as it goes. But if you actually go to section 3.3.6 as it advises, you will get the full explanation. –  Michael Hampton Dec 10 at 16:54
    
@MichaelHampton Because BSD 4.2 uses 0 as broadcast? –  Grezzo Dec 10 at 17:12
1  
@Grezzo Pretty much. You're stuck with this because the 80's just won't die. –  Michael Hampton Dec 10 at 17:16

Sounds like there is a little confusion here with basic networking.

The 'ancient hardware' referred to in one of the responses would not use IP Subnet Zero - using ip address x.x.x.0 with a network set up for a /24 CIDR or 255.255.255.0 subnet mask is a different issue completely.

IP Subnet Zero

  • Older hardware would not use IP Subnet Zero - meaning they would not use the first subnet on a multi-subnet network system. So on a /23 or 255.255.254.0 network the subnet X.X.X.0 and all it's addresses would not be used. Modern routers do not have this restriction but can be configured to use this old model I.E. not use subnet zero, if desired.

Usable host IP addresses on a subnet

  • Basic networking:
    • With /24 i.e. subnet mask of 255.255.255.0
    • x.x.x.0 is reserved as the network address. Routers and routing protocols (EIGRP, RIP2, etc.) use the network address to define network segments for moving packets within and across network network boundaries.
    • x.x.x.255 is reserved for the broadcast address
    • It is common practice to use the addresses .1 or .254 on routers so that leaves 253 usable IP numbers.

Both the network address and the broadcast address are reserved and cannot (by current and previous network standards) be assigned to a device. Using x.x.x.0 for a host address on a /24 system is wrong. Even if Linux lets you use it does not mean that it is right, it just means that Linux thinks that you know what you are doing.

If your system is letting you assign x.x.x.0 as an IP4 address to a host and it appears to be working - chances are that particular host is receiving ALL traffic targeted to ANY device on that network so its networking is probably not working optimally.

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1  
You have said "x.x.x.0 is reserved as the network address (think of it as global address or pointer to the whole subnet)" but not explained what it is actually used for, and therefore not explained why it can't be used for a host –  Grezzo Nov 24 '12 at 8:19
2  
Updated my post. Bottom line is that assigning the network address to a device is against the specs and while you might get away with using it in a small stub network you still run the risk that if you're router gets a software update (or if you change routers) that the device or your whole network may stop functioning properly. –  TheSteven Nov 25 '12 at 10:29

An address with an all-zero host portion refers to the network itself, rather than to any particular host.

Historically, this zero host address has served as an alternative broadcast address, and devices still respond that way.

So, I have to disagree with some other answers: no, zero is not a perfectly usable host address. If you need more than 254 addresses, you have to create a larger subnet.

Look, my Linksys router, whose address is .1 responds to pings of .0. (The netmask is 255.255.255.0, so the last octet corresponds to the host number.)

webserver:~# ping  192.168.1.0
Do you want to ping broadcast? Then -b
webserver:~# ping -b 192.168.1.0
WARNING: pinging broadcast address
PING 192.168.1.0 (192.168.1.0) 56(84) bytes of data.
64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=1.46 ms
64 bytes from 192.168.1.1: icmp_seq=2 ttl=64 time=0.812 ms
64 bytes from 192.168.1.1: icmp_seq=3 ttl=64 time=0.819 ms

If I assigned the .0 address to some host, I wouldn't be able to ping it without the router chiming in with its responses also. And as you can see, some tools like that router's version of ping treat 0 as a broadcast.

You can bend the rules, if you're willing to patch all protocol stacks and other software which is affected by such a bending. Otherwise, stick to the rules.

Case in point.

I worked at a company which designed a network node built in a 14 slot chassis, running numerous independent OS images on several types of cards, all communicating via a backplane. There was a networking setup over the backplane with a convention that 127.X.0.Y is the internal IP address of node Y in slot X, all numbered from 1.

We basically subnetted the loopback address for our own purposes. To make it work, we had to patch the Linux kernel here and there, and IIRC, a little bit of user space.

Since that network was only used within the box, and most programs that require loopback use the 127.0.* network (and in fact the specific address 127.0.0.1) which continued to work normally, everything was cool.

share|improve this answer

Actually the answer is the basic of subnetting. The "all zeros" IP of your subnet combined with the network id is used to calculate where the packet has to be sent to.

In your example you have a subnet of 255.255.255.0. Any device that knows the TCP/IP protocol will use the netmask combined with the IP address to calculate if the packet is destined for the local network (by performing a logical AND operation) or that it has to be sent trough the gateway / router.

So I guess the reason that IP can not be used is because it is already used to "define" the network boundaries together with the netmask by design.

share|improve this answer
    
You say that "Any device that knows the TCP/IP protocol will use the netmask combined with the IP address to calculate if the packet is destined for the local network (by performing a logical AND operation)", but ANDing 83.23.159.0 with 255.255.255.0 and ANDing 83.23.159.1 with 255.255.255.0 produces the same result (83.23.159.0) so using this calculation to figure out if it is destined for the network or router still works even when a host has the .0 address –  Grezzo Nov 23 '12 at 10:32

actually it depends on the netmask, for example, for the network 83.23.159.0/23, 83.23.159.0 is a perfectly usable ip address

share|improve this answer
2  
+1 - 83.23.158.255 would also be a valid host address. People get hung up on the no 0's and no 255's rule without considering the subnet mask. –  joeqwerty Nov 22 '12 at 14:54
    
I am aware that a x.x.x.0 address is valid with a netmask of less that /24, but notice that in my question I specified a /24 netmask, so you haven't answered my question. I wasn't talking about all 0s or 255s in the last quartet/byte, I said all 0s or 255s in the in the "host portion" of the IP address. This takes into account the netmask. –  Grezzo Nov 22 '12 at 17:20
2  
@Grezzo short answer: non-CIDR and legacy stuff breaks with .0, MS just disallows it completely. But IPv6 is imminent so why bother worrying? ;) –  Sammitch Nov 22 '12 at 22:08
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This is not really an answer since the question isn't really about dot notation numerology but about the status/meaning of a 0 host address in any width subnet. –  Kaz Nov 22 '12 at 23:53
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I don't see why this answer has so many up votes. 83.23.159.0/23 isn't "technically" a valid network. 83.23.159.0 with a 23 mask is actually an ip right in the middle of the 83.23.158.0/23 network. The question wasn't "why can't I have .0 at the end of the ip", it was "why can't I use the address with host part all zeros". In your example, the "all host zeros" address would be 83.23.158.0. As mentioned in other answers, this is historically an unusable address (the "network address") and while it may work on some OS's, it is not advisable to try and use it. –  USD Matt Nov 24 '12 at 19:29

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