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Page 316 / 367 Scroll up to view Page 311 - 315
Appendix B IP Addresses and Subnetting
VMG4380-B10A / VMG4325-B10A User’s Guide
316
The following table shows some possible subnet masks using both notations.
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 185
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.
Table 117
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
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
Page 317 / 367
Appendix B IP Addresses and Subnetting
VMG4380-B10A / VMG4325-B10A User’s Guide
317
The following figure shows the company network after subnetting. There are now two sub-
networks,
A
and
B
.
Figure 186
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 118
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
Page 318 / 367
Appendix B IP Addresses and Subnetting
VMG4380-B10A / VMG4325-B10A User’s Guide
318
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.
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
Table 119
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 120
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 121
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 118
Subnet 1 (continued)
IP/SUBNET MASK
NETWORK NUMBER
LAST OCTET BIT
VALUE
Table 122
Eight Subnets
SUBNET
SUBNET
ADDRESS
FIRST ADDRESS
LAST
ADDRESS
BROADCAST
ADDRESS
1
0
1
30
31
2
32
33
62
63
Page 319 / 367
Appendix B IP Addresses and Subnetting
VMG4380-B10A / VMG4325-B10A User’s Guide
319
Subnet Planning
The following table is a summary for subnet planning on a network with a 24-bit network number.
The following table is a summary for subnet planning on a network with a 16-bit network number.
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
Table 122
Eight Subnets (continued)
SUBNET
SUBNET
ADDRESS
FIRST ADDRESS
LAST
ADDRESS
BROADCAST
ADDRESS
Table 123
24-bit Network Number Subnet Planning
NO. “BORROWED”
HOST BITS
SUBNET MASK
NO. SUBNETS
NO. HOSTS PER
SUBNET
1
255.255.255.128 (/25)
2
126
2
255.255.255.192 (/26)
4
62
3
255.255.255.224 (/27)
8
30
4
255.255.255.240 (/28)
16
14
5
255.255.255.248 (/29)
32
6
6
255.255.255.252 (/30)
64
2
7
255.255.255.254 (/31)
128
1
Table 124
16-bit Network Number Subnet Planning
NO. “BORROWED”
HOST BITS
SUBNET MASK
NO. SUBNETS
NO. HOSTS PER
SUBNET
1
255.255.128.0 (/17)
2
32766
2
255.255.192.0 (/18)
4
16382
3
255.255.224.0 (/19)
8
8190
4
255.255.240.0 (/20)
16
4094
5
255.255.248.0 (/21)
32
2046
6
255.255.252.0 (/22)
64
1022
7
255.255.254.0 (/23)
128
510
8
255.255.255.0 (/24)
256
254
9
255.255.255.128 (/25)
512
126
10
255.255.255.192 (/26)
1024
62
11
255.255.255.224 (/27)
2048
30
12
255.255.255.240 (/28)
4096
14
13
255.255.255.248 (/29)
8192
6
14
255.255.255.252 (/30)
16384
2
15
255.255.255.254 (/31)
32768
1
Page 320 / 367
Appendix B IP Addresses and Subnetting
VMG4380-B10A / VMG4325-B10A User’s Guide
320
Configuring IP Addresses
Where you obtain your network number depends on your particular situation. If the ISP or your
network administrator assigns you a block of registered IP addresses, follow their instructions in
selecting the IP addresses and the subnet mask.
If the ISP did not explicitly give you an IP network number, then most likely you have a single user
account and the ISP will assign you a dynamic IP address when the connection is established. If this
is the case, it is recommended that you select a network number from 192.168.0.0 to
192.168.255.0. The Internet Assigned Number Authority (IANA) reserved this block of addresses
specifically for private use; please do not use any other number unless you are told otherwise. You
must also enable Network Address Translation (NAT) on the Device.
Once you have decided on the network number, pick an IP address for your Device that is easy to
remember (for instance, 192.168.1.1) but make sure that no other device on your network is using
that IP address.
The subnet mask specifies the network number portion of an IP address. Your Device will compute
the subnet mask automatically based on the IP address that you entered. You don't need to change
the subnet mask computed by the Device unless you are instructed to do otherwise.
Private IP Addresses
Every machine on the Internet must have a unique address. If your networks are isolated from the
Internet (running only between two branch offices, for example) you can assign any IP addresses to
the hosts without problems. However, the Internet Assigned Numbers Authority (IANA) has
reserved the following three blocks of IP addresses specifically for private networks:
•
10.0.0.0
— 10.255.255.255
•
172.16.0.0
— 172.31.255.255
•
192.168.0.0 — 192.168.255.255
You can obtain your IP address from the IANA, from an ISP, or it can be assigned from a private
network. If you belong to a small organization and your Internet access is through an ISP, the ISP
can provide you with the Internet addresses for your local networks. On the other hand, if you are
part of a much larger organization, you should consult your network administrator for the
appropriate IP addresses.
Regardless of your particular situation, do not create an arbitrary IP address; always follow the
guidelines above. For more information on address assignment, please refer to RFC 1597,
Address
Allocation for Private Internets
and RFC 1466,
Guidelines for Management of IP Address Space.

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