Significance of 255.255.255.255 (/32) Subnet mask ?

Answered Question
Jun 21st, 2012

Hi Experts ,

Here is the scenerio

- The nodes are Web Servers at one of our client site

- Multiple NIC Cards . One of them has a static Private IP Address in 192.168.168.X with /24 subnet mask for communication to a SAN. Two of them are teamed and the Team is assigned with static Public IP Addresses . The preferred IP Address is a public IP address with subnet mask 255.255.255.128 (/25) and there are additional IP address given on the teamed NIC ,most of them with 255.255.255.255 (/32) . What is the significance of giving such a subnet mask ?

Please help.

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Correct Answer by colin.farley about 2 years 11 months ago

This is common web server configuration. Normally webservers will serve many web sites and one way of having multiple sites on a server involves having the web server (IIS, apache, etc) processes bind to unique IPs. The addresses with the 32bit mask are just IP aliases for inbound traffic and you will likely find your web server software configured to use the IPs for the various web sites. If you use Windows 2008 without a specific hotfix beware they may be used as the source address for outbound traffic too.

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Overall Rating: 3.7 (3 ratings)
adnane dakna Thu, 06/21/2012 - 02:30

Hi ,

the general purpose of using mask  comes from the aspect for opmtimizing   IP addressing and best handling routing process , when we configure IP address with mask 255.255.255.255 , we anounce just a unique address (not a network) , this type of mask is using when configure loopback address for management purposes.

Anup Sasikumar Thu, 06/21/2012 - 09:45

Thanks for your reply , Adnane.

Correct me if I am wrong , but lets say we gave an IP Address on an NIC say 192.168.1.10 with the default Class C Subnet mask of 255.255.255.0 . What does it actually imply ? It would only be able to communicate with the nodes in 192.168.1.X range ( without any routing enabled ) , right ? But when you have a /32 that would not be specifying a network and hence it would not be communicating with any nodes right ?

I know its the very basics of networking ,but this /32 is confusing Thats why !

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handoko-wiyanto Thu, 06/21/2012 - 11:49

hi Anup,

interesting what you have here. i never have the chance having /32 ip public on a production servers.

normally use /32 ip for loopack ip, or router id. its for management purpose (access the network device) or routing protocol purpose.

do the additional ip address also public ip address? if yes, then perhaps your isp is running out of ip public?

when you have /24, it means the portion of network bit are 24 bits, this leave us 8 bit or 254 usable address for host portion

when you use /32, it means you are implying on the host itself, it still can be connected to the other host or to the gateway.

regards,

Correct Answer
colin.farley Fri, 06/22/2012 - 00:07

This is common web server configuration. Normally webservers will serve many web sites and one way of having multiple sites on a server involves having the web server (IIS, apache, etc) processes bind to unique IPs. The addresses with the 32bit mask are just IP aliases for inbound traffic and you will likely find your web server software configured to use the IPs for the various web sites. If you use Windows 2008 without a specific hotfix beware they may be used as the source address for outbound traffic too.

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Posted June 21, 2012 at 2:06 AM
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