Zero IP address in last octet

Answered Question
Oct 4th, 2007

Hi Folks,

Can we assign a IP address like 192.168.1.0/32 to a host?

Normally we assign a /32 mask ip address to a loopback interface. I tried to assign this IP address to a loopback interface on a router and it was accepted & pinging.

But my question is can this IP (192.168.1.0/32) assign to a server?

is zero IP address in last octet a valid IP address?

Please help me to understand.

Thanks in advance.

-Amolak

Correct Answer by paul.matthews about 9 years 4 months ago

I'm closer to Kevin's position. with a /32 it is a host route, but as to assigning it to a host - that would be a tentative NO!.

Assigning it to an interface means there is no room in the "subnet" for a router, so giving that address to a host without some assumptions on the part of other devices would not actually function. I suppose it could be linked in with mobile IP, and the host sends a gratuitous arp. The router would need to know to accept that, even though the IP address may be outside the subnet, and to add that /32 to the routing table. Access from the device would need to be by assuming everything to be local and proxy arp.

Where it may be of use though is an internal address for a server (a little like using a loopback on a router) that is multi homed. One interface on one subnet, one interface on another subnet, and the /32 is accessible via either. The would involve a degree of care with the connectivity. For that to have a point, the normal position would be for the server to participate in the routing protocol - eg RIP2 or OSPF. That may not be what the network people like - having an end system participate in routing may be considered a security issue. If one does not want the server to participate, you would need to use (floating) static routes. You would then probably want to connect the server to L3 ports on the switch, so that if the NIC or the cable fails, the network the server is connected to will go down, and take that static out of the routing table.

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Danilo Dy Thu, 10/04/2007 - 05:48

Hi,

Nope, all zeros (00000000) and all ones (11111111) binary in the host portion is invalid.

Regards,

Dandy

Kevin Dorrell Thu, 10/04/2007 - 05:55

Sorry, I disagree. This is a /32, so it is a single isolated host address. In the /32 world there is no such thing as a network address or a broadcast address, so anything goes.

If It is later summarised as part of a /24, /25, etc. then you might have some issues, but even that might work OK. Summarising as part of a /23 etc might well be OK, and, for example, 172.16.3.0/23 is quite a valid host address.

Kevin Dorrell

Luxembourg

Correct Answer
paul.matthews Thu, 10/04/2007 - 06:18

I'm closer to Kevin's position. with a /32 it is a host route, but as to assigning it to a host - that would be a tentative NO!.

Assigning it to an interface means there is no room in the "subnet" for a router, so giving that address to a host without some assumptions on the part of other devices would not actually function. I suppose it could be linked in with mobile IP, and the host sends a gratuitous arp. The router would need to know to accept that, even though the IP address may be outside the subnet, and to add that /32 to the routing table. Access from the device would need to be by assuming everything to be local and proxy arp.

Where it may be of use though is an internal address for a server (a little like using a loopback on a router) that is multi homed. One interface on one subnet, one interface on another subnet, and the /32 is accessible via either. The would involve a degree of care with the connectivity. For that to have a point, the normal position would be for the server to participate in the routing protocol - eg RIP2 or OSPF. That may not be what the network people like - having an end system participate in routing may be considered a security issue. If one does not want the server to participate, you would need to use (floating) static routes. You would then probably want to connect the server to L3 ports on the switch, so that if the NIC or the cable fails, the network the server is connected to will go down, and take that static out of the routing table.

amolak_cisco Thu, 10/04/2007 - 07:53

Hi Guys,

Thanks for your time & valuable inputs.

I got my query resolved.

Regards,

Amolak

ndarnell Thu, 10/04/2007 - 06:09

Dandy,

As its a /32 then the host porttion of the address is in fact all 32 bits therefore is not all 1's or 0's.

Danilo Dy Thu, 10/04/2007 - 06:13

oh, yes /32 sigh. I should have double check that first. Sry, wrong info. Kevin is right.

Regards,

Dandy

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