User's Manual
25
Webthru SWC 306 User’s Guide
E 240 to 255 Reserved for specific utilization
In C class network, the applied number of IP addresses with network masking is as below when you mask on
host ID part (X4: the fourth byte).
Masking on X4 Byte Remark
Host ID
number
25
26 27 28 29 30 31 0 Masking Point
128
(128)
64
(192)
32
(224)
16
(240)
8
(248)
4
(252)
2
(254)
1
(255)
Decimal Number
(Accumulated Value)
Masked
Free Free Free Free Free Free Free 7 digits are free 2
7
= 128
Masked Free Free Free Free Free Free 6 digits are free 2
6
= 64
Masked Free Free Free Free Free 5 digits are free 2
5
= 32
Masked Free Free Free Free 4 digits are free 2
4
= 16
Masked Free Free Free 3 digits are free 2
3
= 8
Masked Free Free 2 digits are free 2
2
= 4
Masked Free 1 digits are free 2
1
= 2
Masked No free digit 2
0
= 1
The most common case is to make subnet through network masking, and it is to divide a network into some
smaller network. If provided IP address is 192.168.1.2, you may divide the whole network into 2 sub-
networks and allow or deny only the IP addresses that belong to one of sub-networks.
With setting as follows, The IP address of 192.168.1.2 is divided into two sub-networks and allow for the IP
address out of the first sub-network to assess Webthru.
• Default Policy: Deny
• IP address: 192.168.1.2
• Masking: 25 (255.255.255.128)
• Then only the IP addresses from 192.168.1.0 to 192.168.1.127 are to access Webthru, while the IP
addresses from 192.168.1.128 to 192.168.1.255 and any other IP address are to be denied accessing
Webthru.
Changing IP address can reverse the result. If you set IP address as 192.168.1.130, only the IP addresses from
192.168.1.128 to 192.168.1.255 are to access Webthru. And the IP addresses from 192.168.1.0 to
192.168.1.127 and any other IP address are to be denied accessing Webthru.
You may refer below table to figure out masking point from network information that is given from your ISP
or network administrator.
Masking Point Masked bit (Network ID) Netmask in decimal number
1 The first bit 128.0.0.0
2 From the first bit to the second bit 192.0.0.0
3 From the first bit to the third bit 224.0.0.0
.
8
9
.
16
.
From the first bit to the 8
th
bit
From the first bit to the 9
th
bit
.
From the first bit to the 16
th
bit
.
255.0.0.0
255.128.0.0
.
255.255.0.0