Subnetting

I am not familiar with network configuratiton, but enough to understand key terms, however, i am not sure about this one: two subnets on network: .255 and .254. I am having issues with client machines connecting with certain printers and especially MS exchange. Why would the previous admin. setup this way and what can I do to resolve to one .255 subnet?
GrizNation23IT DirectorAsked:
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Alan HardistyCo-OwnerCommented:
I would imagine it was a typo.  Subnetting to .254 doesn't achieve anything at all.

Set any clients to .255 and then you shouldn't have any issues.
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Fred MarshallPrincipalCommented:
I presume you mean:
255.255.255.0
AND
255.255.254.0
Is that right?  Well, assuming that it is:

Understand that each device has a subnet mask to tell it what to do.  The subnet masks aren't transmitted in any packets.  So, only the one device knows what it's subnet mask is.

It depends where (on which device) the .254 or the .255 is entered.  
.255 and .254 will have different broadcast addresses and this can make some things not work.

Subnetting with .254 provides 510 usable addresses vs. .255 with 254 usable addresses.
And, depending on the address that uses one or the other subnet mask, it could affect the network address as well.

For example:

10.1.2.3 / 255.255.255.0 has a network address of 10.1.2.0 and broadcast 10.1.2.255
10.1.2.3 / 255.255.254.0 has a network address of   10.1.2.0 and bc 10.1.3.255
10.1.151.3 / 255.255.255.0 has a network address of 10.1.151.0 and bc 10.1.151.255
10.1.151.3 / 255.255.254.0 has a network address of 10.1.150.0 and bc 10.1.151.255

So, in the first two cases the network addresses are the same and the broadcast addresses are different.
In the last two cases the network addresses are different and the broadcast addresses are the same.
That's because of the base address selected in relation to the subnet mask and, thus, the subnet.

I think you will find that the broadcast address is most important.  And, note that either of them can change depending on the absolute IP address of any device (and the subnet mask).  So, you could have devices trying to broadcast to an address that's either assigned to a device or at least to one that *could be* assigned to a device ... and have half the other devices ignore what was intended to be a broadcast packet.

For further reading you might refer to the attached paper and search the internet as well.

How-Subnets-Work-in-Practice.pdf
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ZShaverCommented:
to fully explain subnets better, we have to look at the binary representations of the addresses

e.g. a subnet mask of 255.255.255.0 will look like...

11111111.11111111.11111111.00000000

where the 1's are the network portion of the address and the 0's are available addresses within the network or available hosts

because there are 8 zero's, 2 to the power of 8 or 2^8 = 256, because you cannot use .0 or .255 as an address this is reduced by 2 to get 254 available addresses
valid addresses are from x.x.x.1 to x.x.x.254

.0 is referred to as the network address
.255 is referred to as the broadcast address
these are both reserved and dependent on subnet mask... e.g. the network address is when the host portion of the address is all 0's, and the broadcast address is when the host portion of the address is all 1's.


a subnet mask of 255.255.0.0 looks like this in binary:
11111111.11111111.00000000.00000000

and so 2^16 = 65536, subtracting 2 for the network address and broadcast address this leaves 65534 addresses available.
with a valid range of addresses from x.x.0.1 to x.x.255.254
with x.x.0.0 and x.x.255.255 being reserved addresses for network address and broadcast address

As a more complicated example, let's assume we have a network with a subnet mask of 255.255.254.0 with a network address of 192.168.0.0

In binary the subnet address would be 11111111.11111111.11111110.00000000
and thus you have 2^9 = 512 addresses,
from 192.168.0.1 to 192.168.1.254
with 192.168.0.0 and 192.168.1.255 being reserved
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ZShaverCommented:
If machines are on the same subnet, they are usually connected together using a switch or hub and they can talk directly to each other, as they sense that they are on the same network

If a machine is outside of the subnet, it is accessed through the default gateway/router.

So if your 2 machines are on different subnets, they will usually not be able to communicate because they will try to find each other through the router, rather than accessing each other directly through the switch.


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Fred MarshallPrincipalCommented:
ZShaver is correct to a point:

- If the router has a broader subnet than any of the individual hosts then none of the devices will know the difference ... except for the broadcast address difference.  The router won't  receive any of the host's broadcast packets as such.  But all the hosts will.  Some stuff will work fine and other stuff won't and it's too complicated to try to delineate.

- If an individual host has a broader subnet than the router then the router may well drop packets sent to it.

- If a host has a broader subnet than all other hosts and broader than the router, it may still be able to communicate with the other hosts if it's own address is in the narrower range and because no other computer or router can tell that the subnets are different.  Yet, broadcast packets won't go to the right address.
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GrizNation23IT DirectorAuthor Commented:
I apologize, I was in a rush creating this question.
1. In an Educational school network, however, I am certain two subnet masks were not the intention
     for security, but a administrator that did not have much networking knowledge. Believe he was
      just trying to create different "subnets" not masks.
   
2. Subnet mask are both being sent out on this DHCP server with computers throughout either        
    recieving a .254 or .255 subnet mask, which either gives them access to the Exchange and
    printers or drops them when they recieve a .254 mask.

3. again on a  DHCP server not static.

4. unable to keep clients connected to exchange due to the mask issue.

I hope that makes better sense of my issue.
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Fred MarshallPrincipalCommented:
OK.

I don't understand how "this DHCP server" can send  out more than one set of things .. including the subnet mask.  Can you tell us more about this?
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ZShaverCommented:
i'm not even sure what you mean by ".254" or ".255" subnet mask...
do you mean 255.0.0.0. 255.255.255.0, 255.255.255.255, or what?

a subnet mask of 255.255.255.255 indicates that this machine is the  only one on the local network and all destinations routed through default gateway
this is the case for most if not all dialup, PPP, PPPoE connections

Like fmarshall said however the DHCP server should only be sending out 1 subnet mask. Unless there are 2 different networks there and 2 different dhcp servers.
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GrizNation23IT DirectorAuthor Commented:
ok, I had one dhcp server set as  .255 for subnet and not the attended .254, while the dhcp server had a .254, so we were getting either or. all is perfect now. thanks all for your comments.
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GrizNation23IT DirectorAuthor Commented:
thank you all for your comments, I did learn abit more about subnetting as a result of this question. thank you
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Fred MarshallPrincipalCommented:
The answers were provided in good faith.  It seems untoward to award no points.
The self-answer given is unclear so isn't helpful to the community.
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