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Page 276 / 324 Scroll up to view Page 271 - 275
Appendix B IP Addresses and Subnetting
P-660HN-TxA User’s Guide
276
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 103
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
Table 104
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
Page 277 / 324
Appendix B IP Addresses and Subnetting
P-660HN-TxA User’s Guide
277
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 105
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 106
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 107
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
Page 278 / 324
Appendix B IP Addresses and Subnetting
P-660HN-TxA User’s Guide
278
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.
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
Table 108
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 109
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 279 / 324
Appendix B IP Addresses and Subnetting
P-660HN-TxA User’s Guide
279
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 ZyXEL Device.
Once you have decided on the network number, pick an IP address for your ZyXEL
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
ZyXEL 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 ZyXEL 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.
Page 280 / 324
Appendix B IP Addresses and Subnetting
P-660HN-TxA User’s Guide
280

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