Subnet Mask

[Rocha]

Hi, I have a question about the Subnet Mask 255.255.255.0
If I use any class C IP address, let's say 192.168.1.0 and the default subnet mask 255.255.255.0, how many hosts and subnets do I have and what is my IP range of addresses??
Would I do, 2^0 = 1 for subnets and 2^8-2=254 for hosts and for the block size 256 - 255 = 1??
The book doesn't give this example, It starts with the mask 255.255.255.128 for class C IP addresses.

Thanks,

Rocha

[MiniMatt]

Yep, exactly that. Your broadcast address is 1.255, your network address is 1.0 - you've still lost the 2 host addresses as per normal rules.

Only subnet that kind of has special rules is 255.255.255.254 - you won't ever see that because that'd result is a grand total of 0 hosts. However you will see 255.255.254.0.

Oh and I suppose 255.255.255.128 would only work if you've got ip subnet zero turned on (which it is by default, so unlikely to be a problem, but one to be aware of).

[crissa]

[...]

Only subnet that kind of has special rules is 255.255.255.254 - you won't ever see that because that'd result is a grand total of 0 hosts. However you will see 255.255.254.0.

[...]

Hello,
I saw 255.255.255.254 used for a transit network in a Juniper documenation ...
But so far I never saw it in real life.
Bye, Tore

[Big Evil]

Funny enough we use the 255.255.255.254 at work and most UK ISP's do for home users.

[HermeszData]

Funny enough we use the 255.255.255.254 at work and most UK ISP's do for home users.

It is not unusual to see ISPs here in the US use the *.*.*.252 mask in class C implimentations. I have never run across a *.*.*.254 mask.

HughesNet as an example (unless you subscribe for a Static IP Address) uses 192.168.0.1 255.255.255.252 for all their subscribers' IP Address. (Basic Internetworking Subnetting 101). I have also seen several of the Cable companies do the same thing.

How can an ISP use a mask of 255.255.255.254 as this does not allow for:
1) A Subnetwork
2) A host Address
3) A default Gateway
4) A Broadcast Address

Unless, I am missing something, we have to have all the above to be able to communicate and have an Internetwork.

I am of course assuming a "Fat Finger" error in the above.

John

[MiniMatt]

Well I stand corrected :D Was aware there was an RFC around for it but hadn't seen it in anger. I guess "what you won't see in production" relies very much on what production areas you work in :)