Zyxel NBG5615 Router Manual PDF (Setup & Configuration Guide)

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Appendix B IP Addresses and Subnetting

NBG5715 User’s Guide

186

Figure 123

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 74

IP Address Network Number and Host ID Example

1ST OCTET:

(192)

2ND

OCTET:

(168)

3RD

OCTET:

(1)

4TH OCTET

(2)

IP Address (Binary)

11000000

10101000

00000001

00000010

Subnet Mask (Binary)

11111111

00000000

Network Number

11000000

10101000

00000001

Host ID

00000010

Page 187 / 252

Appendix B IP Addresses and Subnetting

NBG5715 User’s Guide

187

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 75

Subnet Masks

BINARY

DECIMAL

1ST

OCTET

2ND

OCTET

3RD

OCTET

4TH OCTET

8-bit mask

11111111

00000000

255.0.0.0

16-bit mask

11111111

00000000

255.255.0.0

24-bit mask

11111111

00000000

255.255.255.0

29-bit mask

11111111

11111000

255.255.255.248

Table 76

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

255.255.255.248

3 bits

2

3

– 2

6

Table 77

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 188 / 252

Appendix B IP Addresses and Subnetting

NBG5715 User’s Guide

188

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 124

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 77

Alternative Subnet Mask Notation (continued)

SUBNET MASK

ALTERNATIVE

NOTATION

LAST OCTET

(BINARY)

LAST OCTET

(DECIMAL)

Page 189 / 252

Appendix B IP Addresses and Subnetting

NBG5715 User’s Guide

189

Figure 125

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 78

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 190 / 252

Appendix B IP Addresses and Subnetting

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190

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 79

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 80

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 81

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 82

Eight Subnets

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

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