Using the Ellis
Methodology of Subnetting “EMS”
“EMS” Simplified Method Class C
This is a simplified method of subnetting by remembering a few basic rules. Network Host
2. In
basic subnetting, you don’t use the first IP value as it’s the network address,
and you don’t use the last IP value as it’s the broadcast address.
3. Identify
the value of each bit in an octet beginning with 1 and multiplying it by 2 for
each bit place in an octet as illustrated below in line 1 below.
4. Subtract
your first and last IP values from each bit value in line 1 or take 2 from each
place, as illustrated in line 2 below. If you can’t take 2 from 1, leave it
blank.
5. Reverse
the numbers starting with 0 and place it under 126 of line 2 and continue until
126 is under the zero of line 2, as illustrated in line 3 below.
6. As
you borrow bits from the left you decrease the number of available hosts as you
increase the number of available networks. So, the hosts, line 2, decrease as
the sub-networks, line 3, increase.
7. Now
for any Cisco IOS above 12.0 remember to add two to each value in line 3 as we
gain those networks back in the new version. If use an older IOS, leave the
values as indicated in the chart.
8. Line
or row 1 identifies the quantity of IP addresses for each subnet in total. Line or row 2 identifies how many hosts can
exist in the subnetted range, and line or row 3 identifies how many subnets can
exist in the subnetted range.
9. A
way to remember what each line represents use the first letter from each line
I, H and S and just remember “I Hate Sub-Netting.”
Class C Subnetting Guide
Line 1. “IP’s”
128 64 32 16 8 4 2 1
Line 2. “Hosts” 126 62 30 14 6 2 0
Line
3. “Subnets” 0 2 6 14 30 62 126
Now that you have your Class C “Cheat Sheet,” remember that by using
this guide you can subnet any Class C IP address. Remember that the values on
line 2 are the number of usable hosts and the values on line 3 are the number
of usable sub-networks created when borrowing the bits across the values on
line 1 from left to right. That is because we took away both the network IP and
broadcast IP from each. You can add two back to each value in lines 2 and 3 to
know how many actual hosts and networks are actually created. This is for IOS
11.0 and older.
We begin by giving up bits from left to right starting with the
128 bit and moving right to get the subnet range we want. If we want 30 host
computers on each of the six subnetworks, we would give up the 128, the 64, and
the 32-bit values of line 1; which is the host values of that octet. This means,
we would borrow those three bits. We would identify this by drawing a line down
from the 32 past the 30 to the 6 as indicated in the example below.
To get the subnet mask, simply add the values of each bit you
borrowed from left to right on line 1. If you add 128 and 64 and 32 together you
get 224. Now the new subnet mask is 255.255.255.224
Example
In this example,
we are borrowing the first 3 bits in the 4th octet to give up to
subnetting.
1. 4th Octet bit representation 128 64 32 | 16 8 4 2 1 “IP’s”
2. Subtract First & Last 126 62 30 | 14 6 2 0 “Hosts”
3. Reverse of Line 2 0 2 6 | 14 30 62 126 “Subnets”
Again, if you wanted to create a subnet with 30 hosts on each of six
subnetworks as indicated above, you would borrow the first three bits, always
from the left.
We started with a Class C default mask of 255.255.255.0 To get a
new subnet mask we add the values of the three bits that are used: 128 + 64 +
32 = 224.
That would give us a subnet mask of 255.255.255.224 where the 255
represents the value for the other octets.
Next to get the allocated IP addresses for each subnet, notice
the number 32 in the column you took the bits to. This identifies how many IP
are in each subnet. Each of the subnets are in groups of 32 IP addresses as indicated
by the table below.
This is created by starting with 0, go vertically “down” in
groups of 32 as indicated here:
0
32
64
96
Etc.
to 255 as shown in Table 1 below
This is the first bit value in each of the new subnet ranges. Now
fill in the last value for each subnet range by realizing that it will be one
less than the start of the next range as indicated here:
0------------31
32----------63
64----------95
96---------127
Etc.
to 255 as shown in Table 1 below
Table 1:
Subnet
|
Network Number
|
Host IP Range
|
Usable IP Range
|
Subnet 1
|
192.168.1
|
0-------------31
|
1-30 “First Usable Subnet Range”
|
Subnet 2
|
192.168.1
|
32------------63
|
33-62
|
Subnet 3
|
192.168.1
|
64------------95
|
65-94
|
Subnet 4
|
192.168.1
|
96-----------127
|
97-126
|
Subnet 5
|
192.168.1
|
128---------159
|
129-158
|
Subnet 6
|
192.168.1
|
160---------191
|
161-190
|
Subnet 7
|
192.168.1
|
192---------223
|
193-222
|
Subnet 8
|
192.168.1
|
224---------255
|
225-254 “Last Usable Subnet Range”
|
Remember that we can’t use either the first value in a network
range (it’s reserved as the network number value) or the last value in a
network range (it’s reserved as the network broadcast value).
So, from Table 1 above, we can see that:
·
The IP address, 192.168.1.1 in subnet range 1 is
the first usable address in that subnet range.
·
The IP address, 192.168.1.30 in subnet range 1,
is the last usable address in that subnet range.
·
The IP address, 192.168.1.225 in subnet range 8,
is the first usable address in that subnet range.
·
The IP address, 192.168.1.254 in subnet range 8,
is the last usable address in that subnet range.
The same is true for all of the other ranges between them.
Note that the values in the host IP range are the values
available for the host addresses in the 4th octet. Also, those
values identify both the network value and the broadcast value for each of the
subnets.
The values indicated in the usable IP range are the specific IP
addresses usable only on that subnet.
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