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Name _______________________
Lyons Township High School
Networking Essentials (Net+)
Borrowing bits to create subnets
Borrowing bits to create subnets
Why?



bits are borrowed to allow for the creation of subnets.
by doing so, broadcasts can be localized and not effect efficiency on other parts of a network
also allows for easier administration of LANs by designating certain ranges of IPs for certain rooms.
How?




Remember that all IP addresses have four octets, each octet number containing eight bits.
Class A addresses designate the first octet as that of the network (non-changeable).
Class B addresses designate the first AND second octets as the network.
Class C addresses designate the first, second, AND third.
Class A Network
Class B Network
Class C Network


Network / Host
Determination
N.H.H.H
N.N.H.H
N.N.H.H
Value of
first octet
0-127
128-191
192-223
Bits are borrowed from the host portion of the IP address.
The number of bits borrowed depends on how many hosts or subnets you want to create.
Network
Type
# octets from
which you can
borrow bits
Class A
3
Number of bits
you can borrow
(# octets * 8) –
2
22 (3 * 8 – 2)
Class B
Class C
2
1
14 (2 * 8 – 2)
6 (1 * 8 – 2)
Max. number of
subnets you can
create
4,194,302 (2^22 –
2)
16,382 (2^14 – 2)
62 (2^6 – 2)
Bits
borrowed
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Class A
#
# hosts
subnets
2 4,194,302
6 2,097,150
14 1,048,574
30
524,286
62
262,142
126
131,070
254
65,534
510
32,766
1,022
16,382
2,046
8,190
4,094
4,094
8,190
2,046
16,382
1,022
32,766
510
65,534
254
131,070
126
262,142
62
524,286
30
1,048,57
14
4
2,097,15
6
0
4,194,30
2
2
Class B
#
# hosts
subnets
2 16,382
6
8,190
14
4,094
30
2,046
62
1,022
126
510
254
254
510
126
1,022
62
2,046
30
4,094
14
8,190
6
16,382
2
Class C
# subnets # hosts
2
6
14
30
62
62
30
14
6
2
Determining subnet, broadcast, subnet mask, and range of IP addresses for a subnet
An IP address is first designated with its Class (A, B, or C); you then need to calculate the subnet mask. The
subnet mask is nothing more than the IP address divided into its three parts (Network, Subnet, and Host).
The subnet part is designated by the bits borrowed.
134.15.10.76, with an 10-bit subnet mask
Step one – convert the IP address (network and host portions) into binary:
134.15.
10
.
76
becomes
134.15.00001010.01001100
Step two – divide the binary section into subnet and host portions (using number of bits borrowed):
134.15.00001010.01 001100
NETWORK
SUBNET
HOST
FYI – the host device is on the 41st subnet (0000101001 in decimal = 41), and is the
the 12th device (001100 in decimal = 12) on the subnet.
Step three – determine the subnet’s network address (host portion has all 0’s):
134.15.00001010.01000000
or
134.15.10.64
Step four – determine the subnet’s broadcast address (host portion has all 1’s):
134.15.00001010.01111111
or
134.15.10.127
Step five – determine the range of available IP addresses for ports and computers on that subnet:
Network address
First available
IP address for
device/port
Last available
IP address for
device/port
134.15.10.64
134.15.10.65
134.15.10.126
Broadcast
address
134.15.10.127
134.15.10.65 – 134.15.10.126
Step six – determine subnet mask (network & subnet portions are all 1’s; host portion is all 0’s):
255.255.11111111.11000000
or
255.255.255.192
Name _______________________
Lyons Township High School
Networking Essentials (Net+)
Knowing nothing more than the following two pieces of information, fill out the following table.
IP address:
38.0.17.36
You will need to have 35 hosts in each subnet.
Class of IP address (A, B, or C)
Number of bits to borrow
Subnet address
Subnet broadcast address
First available IP address for a device
Last available IP address for a device
Subnet mask
Which device this is on the subnet (first, second,
etc.)
Which subnet this is (first, second, etc.)
ANSWER KEY
Lyons Township High School
Networking Systems (CCNA)
Borrowing bits to create subnets (Quiz)
Knowing nothing more than the following two pieces of information, fill out the following table.
IP address:
38.0.17.36
You will need to have 35 hosts in each subnet.
Class of IP address (A, B, or C)
Number of bits to borrow
Subnet address
Subnet broadcast address
A
18
38.0.17.0
38.0.17.63
First available IP address for a device
38.0.17.1
Last available IP address for a device
38.0.17.62
Subnet mask
255.255.255.192
Which device this is on the subnet (first, second,
etc.)
36th
Which subnet this is (first, second, etc.)
68th
38.00000000.00010001.00100100