Tuesday, May 22, 2007

Subnetting for Exchange Servers & for FUN

One of the most fun staff Subletting. I have decided to post this question here so that my students can post the answers here. Remember the question, what is an IP address. I have seen some people spending 10 minutes to explain this question. The simple answer would be "It is 32 Bits Binary number. What is a subnet mask than. The answer would be IT is 32 bits binary number as well. Him sound like they are twins, so what does subnet mask do? The answer would be "it divides an IP address into two distinct groups", network portion and host portion. Why is this so important than? Well the answer would be,

IP Address=32Bits Binary number

Subnet mask=32Bits Binary number

IP Address=Network+Host, The subnet Mask determines the Network portion as well as Host portion of the IP Address

If I tell you I live in Washington BLVD and if you live in Washington BLVD, do you think if we are we neighbors? (Assuming Washington Blvd is walkable distance)

Well the answer is maybe or maybe not.

I need my students explain the rest of the story

Here is the question



  1. What is IP address
  2. What is Subnet mask
  3. What is Default Gateway
  4. What is First Network
  5. What is Default Gateway
  6. What is First Usable IP Address
  7. What is Last Usable IP Address
  8. What is Broadcast IP Address
  9. How many Networks
  10. How many Hosts in each Network

User2 (Located on Network 2th)

  1. What is IP ADDRESS?
  2. What is Subnet Mask?
  3. What is Default Gateway?

Oz Ozugurlu


Oz Ozugurlu said...

I ask this question often in interviews,
What is /27 subnet mask? You would be surprised to see the answers people are coming up with. I understand and don’t understand being in IT 10 some years and not getting this right?
What is that about?

Scott said...

/27 = (8networks 30host per network 240 total hosts)

First User
IP Address =
Subnet Mask =
Default Gateway =
5th network.
1st network
Default Gateway
First available
Last available

Second User
8 networks
30 host per network
IP Address =
Subnet Mask =
Default Gateway =

Totally from memory

Oz Ozugurlu said...

Himm I will buy you a berr for sure,
I need rest of the class come back here and speak up
Guys am I buying Beer for Scott or not?

Will said...

First User is in the 5th network

IP Address:
Subnet Mask:
Default Gateway:
Network ID:
First Network:
First Usable IP:
Last Usable IP:
Broadcast IP:
Total Networks: 8
Hosts in each Network: 30

User on the 2nd Network

IP address:
Subnet Mask:
Default Gateway:
Network ID:

Will said...

User on the 2nd Network

IP address:
Subnet Mask:
Default Gateway:
Network ID:

Oz Ozugurlu said...

/27 means 27 Bits Turned on, in total 32 Bits, we have left 5 Bits to play with.

3Bits Network
5Bits for Host

If we do the Math 2*N=Networks, 2*H – 2 = Hosts in each network (why are we minus the number 2 from hosts? Because every network will need a unique IP address to identify the network itself and every network will need a Broadcast IP as well (remember DHCP Process)
2*3=H  8N
2*5-2=H  30H
We used 3Bits which are, 128+64+32=224
Incremental is the smallest Bit which is 32, we will increment this until we go over 255
Network as below ( 0,32,64,96,128,160,192,224,) 256 won’t exist in 8 Bits Binary, max number is all bits (8Bits) turned on, 11111111=255 Network1 Network2 Network3 Network4 Network5 Network6 Network7 Network8 Nopeeeeeee 1 above 255, so it won’t work in 8Bits Binary world
In the Example the Given IP is so we expect the see the PC1/USER1 in the same network as this node is. The number 135 fits into the Network .128, because 135 is a number between 128 and 160 networks

This network will be \27 FGW (respect to a CISCO) FUIP (First usable IP) LUIP (last Usable IP) BIP (Broadcast IP) (Next network)
PC1 IP can be assigned any IP range between

Rodrigo said...


Solution to Problem #1:

IP address of network =
Subnet mask =
Default gateway =
1st network =
Default gateway of 1st network =

1st usable IP address =
last usable IP address =
Broadcast IP address =
Total number of networks = 8
Number of host per network = 30

Solution to Problem #2:

User on 2nd network:

IP address =
Subnet mask =
Default gateway =