Subnetting Hints:
IPv4 Address Format:
4 Bytes / 4 Octets
32 Bits
xxxx . xxxx . xxxx . xxxx
11111111 . 11111111 . 11111111 . 11111111
1st
Octet
2nd
Octet
3rd
Octet
4th
Octet
255 . 255 . 255 . 255
VLSM
Mask
128
192
224
240
248
252
254
255
128
64
32
16
8
4
2
1
/25
/26
/27
/28
/29
/30
/31
/32
Block
Size
CIDR
4th Octect
Remember, subnetting is simply creating networks by “borrowing” bits.
2n-2 = number of subnets created
2n-2 = number of hosts per subnet
n = number of host bits borrowed
n = remaining host bits after creating new mask
You can only subnet Class A, B, & C networks.
At least 2 host bits must remain after borrowing host bits to create a subnet.
AND
0
0
0
0
1
0
1
0
0
1
1
1
OR
0
0
0
0
1
1
1
0
1
1
1
1
Exercise 1- Reverse Engineering an IP Address
Why might you want to reverse engineer:
Is the IP Address Valid?
What is the Network Address?
What is the Network Range?
How to Reverse Engineer:
1. Convert CIDR prefix to doted decimal subnet mask
2. Convert subnet mask to an inverse subnet mask
3. AND the IP address to the subnet mask to determine to network address (lowest address of the
network range)
4. OR the inverse subnet mask to the network address to determine the broadcast address (highest
address of the network range)
5. Network address + 1 = 1st valid host IP address
6. Broadcast address – 1 = Last valid IP host address
Host IP Address:
192.0.0.0/20
Convert CIDR prefix /20 to a doted decimal subnet mask:
/20 = 11111111.11111111.11110000.00000000 = 255.255.240.000
Convert subnet mask to an inverse mask (subtract from all 1’s):
255.255.255.255
11111111.11111111.11111111.11111111
255.255.240.000
11111111.11111111.11110000.00000000
000.000.015.255
00000000.00000000.00001111.11111111 = 0.0.15.255
AND the IP address to the subnet mask to determine the network address:
255.255.240.000
11111111.11111111.11110000.00000000
192.000.000.000
11000000.00000000.00000000.00000000
11000000.00000000.00000000.00000000 = 192.0.0.0
OR the inverse mask to the network addresses to obtain the broadcast address:
192.000.000.000
11000000.00000000.00000001.00000000
000.000.015.255
00000000.00000000.00001111.11111111
11000000.00000000.00001111.11111111 = 192.0.15.255
First valid host:
Network address 192.0.0.0 + 1 = 192.0.0.1
Last valid host:
Broadcast address 192.0.15.255– 1 = 192.0.15.254
Thus:
The Network Address is:
The Host Range is:
The Broadcast Address is:
192.0.0.0
192.0.0.1 to 192.0.15.254
192.0.15.255
Remember the Powers of 2:
128
64
32
16
8
4
2
1
Exercise 2 – Practical IP Address Scheme Design
“Developing an IP Address Plan”
You have been contracted to provide a new network infrastructure design for KSBE AM/FM/TV. The current system is
based upon legacy hubs and an out-of-production router that is no longer supported. Internet access is provided by
several dial-up modems attached to select hosts. All existing network infrastructure will be retired. It is desirable to use
the existing CAT-5 cabling within the facility.
An ISP has also been selected based upon a “trade-out” agreement with the station. The ISP has agreed to provide
public IP address space consisting of a 192.168.100.0/25 subnet for the exclusive use of KSBE. The exact subnet size
requirements are to be determined by the network design. All hosts are to have Internet access, but KSBE desires to
minimize the subnet size in order to minimize the trade-out provisions with the ISP. IP address allocation should
accommodate all existing hosts with the following functional areas of the station:
o
o
o
Sales
Engineering
Production
35 hosts exist today
17 hosts exist today
27 hosts exist today
A minimum growth factor of at least 12% should be used in any IP address allocation plans. In addition, it is desirable to
allocate the IP address space based upon the specified functional areas. Future enhancements may include security
provisions to limit access to hosts between the functional areas and/or the public internet.
How might you design this network’s IP Address plan?
What is the IP address plan for the proposed network?
IP Subnet
Requirements:
S0 - Sales
S1 - Engineering
S2 - Production
Other?
Current
Hosts
12% Growth
35
17
27
40
20
30
Total IP Address Space Required: _________
Network: ____________________________
IP Address:
Subnet Mask:
Subnet Number:
First IP Address:
Broadcast IP Address:
Last IP Address:
Network: ____________________________
IP Address:
Subnet Mask:
Subnet Number:
First IP Address:
Broadcast IP Address:
Last IP Address:
Network: ____________________________
IP Address:
Subnet Mask:
Subnet Number:
First IP Address:
Broadcast IP Address:
Last IP Address:
Network: ____________________________
IP Address:
Subnet Mask:
Subnet Number:
First IP Address:
Broadcast IP Address:
Last IP Address:
Subnet Size Assigned
What additional IP configuration
information is required to configure hosts
on this network?