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Page 266 / 320 Scroll up to view Page 261 - 265
Appendix B IP Addresses and Subnetting
AMG1302/AMG1202-TSeries User’s Guide
266
Figure 159
Network Number and Host ID
How much of the IP address is the network number and how much is the host ID varies according
to the subnet mask.
Subnet Masks
A subnet mask is used to determine which bits are part of the network number, and which bits are
part of the host ID (using a logical AND operation). The term “subnet” is short for “sub-network”.
A subnet mask has 32 bits. If a bit in the subnet mask is a “1” then the corresponding bit in the IP
address is part of the network number. If a bit in the subnet mask is “0” then the corresponding bit
in the IP address is part of the host ID.
The following example shows a subnet mask identifying the network number (in bold text) and host
ID of an IP address (192.168.1.2 in decimal).
By convention, subnet masks always consist of a continuous sequence of ones beginning from the
leftmost bit of the mask, followed by a continuous sequence of zeros, for a total number of 32 bits.
Subnet masks can be referred to by the size of the network number part (the bits with a “1” value).
For example, an “8-bit mask” means that the first 8 bits of the mask are ones and the remaining 24
bits are zeroes.
Table 94
Subnet Masks
1ST OCTET:
(192)
2ND
OCTET:
(168)
3RD
OCTET:
(1)
4TH OCTET
(2)
IP Address (Binary)
11000000
10101000
00000001
00000010
Subnet Mask (Binary)
11111111
11111111
11111111
00000000
Network Number
11000000
10101000
00000001
Host ID
00000010
Page 267 / 320
Appendix B IP Addresses and Subnetting
AMG1302/AMG1202-TSeries User’s Guide
267
Subnet masks are expressed in dotted decimal notation just like IP addresses. The following
examples show the binary and decimal notation for 8-bit, 16-bit, 24-bit and 29-bit subnet masks.
Network Size
The size of the network number determines the maximum number of possible hosts you can have
on your network. The larger the number of network number bits, the smaller the number of
remaining host ID bits.
An IP address with host IDs of all zeros is the IP address of the network (192.168.1.0 with a 24-bit
subnet mask, for example). An IP address with host IDs of all ones is the broadcast address for that
network (192.168.1.255 with a 24-bit subnet mask, for example).
As these two IP addresses cannot be used for individual hosts, calculate the maximum number of
possible hosts in a network as follows:
Notation
Since the mask is always a continuous number of ones beginning from the left, followed by a
continuous number of zeros for the remainder of the 32 bit mask, you can simply specify the
number of ones instead of writing the value of each octet. This is usually specified by writing a “/”
followed by the number of bits in the mask after the address.
For example, 192.1.1.0 /25 is equivalent to saying 192.1.1.0 with subnet mask 255.255.255.128.
The following table shows some possible subnet masks using both notations.
Table 95
Subnet Masks
BINARY
DECIMAL
1ST
OCTET
2ND
OCTET
3RD
OCTET
4TH OCTET
8-bit mask
11111111
00000000
00000000
00000000
255.0.0.0
16-bit mask
11111111
11111111
00000000
00000000
255.255.0.0
24-bit mask
11111111
11111111
11111111
00000000
255.255.255.0
29-bit mask
11111111
11111111
11111111
11111000
255.255.255.248
Table 96
Maximum Host Numbers
SUBNET MASK
HOST ID SIZE
MAXIMUM NUMBER OF HOSTS
8 bits
255.0.0.0
24 bits
2
24
– 2
16777214
16 bits
255.255.0.0
16 bits
2
16
– 2
65534
24 bits
255.255.255.0
8 bits
2
8
– 2
254
29 bits
255.255.255.24
8
3 bits
2
3
– 2
6
Table 97
Alternative Subnet Mask Notation
SUBNET MASK
ALTERNATIVE
NOTATION
LAST OCTET
(BINARY)
LAST OCTET
(DECIMAL)
255.255.255.0
/24
0000 0000
0
255.255.255.128
/25
1000 0000
128
255.255.255.192
/26
1100 0000
192
Page 268 / 320
Appendix B IP Addresses and Subnetting
AMG1302/AMG1202-TSeries User’s Guide
268
Subnetting
You can use subnetting to divide one network into multiple sub-networks. In the following example
a network administrator creates two sub-networks to isolate a group of servers from the rest of the
company network for security reasons.
In this example, the company network address is 192.168.1.0. The first three octets of the address
(192.168.1) are the network number, and the remaining octet is the host ID, allowing a maximum
of 2
8
– 2 or 254 possible hosts.
The following figure shows the company network before subnetting.
Figure 160
Subnetting Example: Before Subnetting
You can “borrow” one of the host ID bits to divide the network 192.168.1.0 into two separate sub-
networks. The subnet mask is now 25 bits (255.255.255.128 or /25).
The “borrowed” host ID bit can have a value of either 0 or 1, allowing two subnets; 192.168.1.0 /25
and 192.168.1.128 /25.
The following figure shows the company network after subnetting. There are now two sub-
networks,
A
and
B
.
255.255.255.224
/27
1110 0000
224
255.255.255.240
/28
1111 0000
240
255.255.255.248
/29
1111 1000
248
255.255.255.252
/30
1111 1100
252
Table 97
Alternative Subnet Mask Notation (continued)
SUBNET MASK
ALTERNATIVE
NOTATION
LAST OCTET
(BINARY)
LAST OCTET
(DECIMAL)
Page 269 / 320
Appendix B IP Addresses and Subnetting
AMG1302/AMG1202-TSeries User’s Guide
269
Figure 161
Subnetting Example: After Subnetting
In a 25-bit subnet the host ID has 7 bits, so each sub-network has a maximum of 2
7
– 2 or 126
possible hosts (a host ID of all zeroes is the subnet’s address itself, all ones is the subnet’s
broadcast address).
192.168.1.0 with mask 255.255.255.128 is subnet
A
itself, and 192.168.1.127 with mask
255.255.255.128 is its broadcast address. Therefore, the lowest IP address that can be assigned to
an actual host for subnet
A
is 192.168.1.1 and the highest is 192.168.1.126.
Similarly, the host ID range for subnet
B
is 192.168.1.129 to 192.168.1.254.
Example: Four Subnets
The previous example illustrated using a 25-bit subnet mask to divide a 24-bit address into two
subnets. Similarly, to divide a 24-bit address into four subnets, you need to “borrow” two host ID
bits to give four possible combinations (00, 01, 10 and 11). The subnet mask is 26 bits
(11111111.11111111.11111111.
11
000000) or 255.255.255.192.
Each subnet contains 6 host ID bits, giving 2
6
- 2 or 62 hosts for each subnet (a host ID of all
zeroes is the subnet itself, all ones is the subnet’s broadcast address).
Table 98
Subnet 1
IP/SUBNET MASK
NETWORK NUMBER
LAST OCTET BIT
VALUE
IP Address (Decimal)
192.168.1.
0
IP Address (Binary)
11000000.10101000.00000001.
00
000000
Subnet Mask (Binary)
11111111.11111111.11111111.
11
000000
Subnet Address:
192.168.1.0
Lowest Host ID: 192.168.1.1
Broadcast Address:
192.168.1.63
Highest Host ID: 192.168.1.62
Page 270 / 320
Appendix B IP Addresses and Subnetting
AMG1302/AMG1202-TSeries User’s Guide
270
Example: Eight Subnets
Similarly, use a 27-bit mask to create eight subnets (000, 001, 010, 011, 100, 101, 110 and 111).
The following table shows IP address last octet values for each subnet.
Table 99
Subnet 2
IP/SUBNET MASK
NETWORK NUMBER
LAST OCTET BIT
VALUE
IP Address
192.168.1.
64
IP Address (Binary)
11000000.10101000.00000001.
01
000000
Subnet Mask (Binary)
11111111.11111111.11111111.
11
000000
Subnet Address:
192.168.1.64
Lowest Host ID: 192.168.1.65
Broadcast Address:
192.168.1.127
Highest Host ID: 192.168.1.126
Table 100
Subnet 3
IP/SUBNET MASK
NETWORK NUMBER
LAST OCTET BIT VALUE
IP Address
192.168.1.
128
IP Address (Binary)
11000000.10101000.00000001.
10
000000
Subnet Mask (Binary)
11111111.11111111.11111111.
11
000000
Subnet Address:
192.168.1.128
Lowest Host ID: 192.168.1.129
Broadcast Address:
192.168.1.191
Highest Host ID: 192.168.1.190
Table 101
Subnet 4
IP/SUBNET MASK
NETWORK NUMBER
LAST OCTET BIT VALUE
IP Address
192.168.1.
192
IP Address (Binary)
11000000.10101000.00000001.
11
000000
Subnet Mask (Binary)
11111111.11111111.11111111.
11
000000
Subnet Address:
192.168.1.192
Lowest Host ID: 192.168.1.193
Broadcast Address:
192.168.1.255
Highest Host ID: 192.168.1.254
Table 102
Eight Subnets
SUBNET
SUBNET
ADDRESS
FIRST ADDRESS
LAST
ADDRESS
BROADCAST
ADDRESS
1
0
1
30
31
2
32
33
62
63
3
64
65
94
95
4
96
97
126
127
5
128
129
158
159
6
160
161
190
191
7
192
193
222
223
8
224
225
254
255

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