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Appendix B IP Addresses and Subnetting
NBG-419N User’s Guide
211
The following figure shows the company network after subnetting. There are now two sub-
networks,
A
and
B
.
Figure 150
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 85
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 212 / 292
Appendix B IP Addresses and Subnetting
NBG-419N User’s Guide
212
Example: Eight Subnets
Similarly, use a 27-bit mask to create eight subnets (000, 001, 010, 011, 100, 101, 110 and 111).
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 86
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 87
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 88
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 85
Subnet 1 (continued)
IP/SUBNET MASK
NETWORK NUMBER
LAST OCTET BIT
VALUE
Page 213 / 292
Appendix B IP Addresses and Subnetting
NBG-419N User’s Guide
213
The following table shows IP address last octet values for each subnet.
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.
Table 89
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
Table 90
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 91
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
Page 214 / 292
Appendix B IP Addresses and Subnetting
NBG-419N User’s Guide
214
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 NBG-419N.
Once you have decided on the network number, pick an IP address for your NBG-419N 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 NBG-419N 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 NBG-419N 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.
13
255.255.255.248 (/29)
8192
6
14
255.255.255.252 (/30)
16384
2
15
255.255.255.254 (/31)
32768
1
Table 91
16-bit Network Number Subnet Planning (continued)
NO. “BORROWED”
HOST BITS
SUBNET MASK
NO. SUBNETS
NO. HOSTS PER
SUBNET
Page 215 / 292
Appendix B IP Addresses and Subnetting
NBG-419N User’s Guide
215
IP Address Conflicts
Each device on a network must have a unique IP address. Devices with duplicate IP addresses on
the same network will not be able to access the Internet or other resources. The devices may also
be unreachable through the network.
Conflicting Computer IP Addresses Example
More than one device can not use the same IP address. In the following example computer
A
has a
static (or fixed) IP address that is the same as the IP address that a DHCP server assigns to
computer
B
which is a DHCP client. Neither can access the Internet. This problem can be solved by
assigning a different static IP address to computer
A
or setting computer
A
to obtain an IP address
automatically.
Figure 151
Conflicting Computer IP Addresses Example
Conflicting Router IP Addresses Example
Since a router connects different networks, it must have interfaces using different network
numbers. For example, if a router is set between a LAN and the Internet (WAN), the router’s LAN
and WAN addresses must be on different subnets. In the following example, the LAN and WAN are
on the same subnet. The LAN computers cannot access the Internet because the router cannot
route between networks.
Figure 152
Conflicting Router IP Addresses Example

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