Table 132 Subnet Masks; Table 133 Maximum Host Numbers; Table 134 Alternative Subnet Mask Notation - ZyXEL Communications P-660HN User Manual

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.

Table 132 Subnet Masks

8-bit mask
16-bit mask
24-bit mask
29-bit mask
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:

Table 133 Maximum Host Numbers

SUBNET MASK
8 bits 255.0.0.0
16 bits 255.255.0.0
24 bits 255.255.255.0
29 bits 255.255.255.248 3 bits
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 134 Alternative Subnet Mask Notation

SUBNET MASK
255.255.255.0
255.255.255.128
P-660HN-Fx User's Guide
BINARY
1ST 2ND
OCTET OCTET
11111111 00000000
11111111 11111111
11111111 11111111
11111111 11111111
HOST ID SIZE
24 bits
16 bits
8 bits
ALTERNATIVE LAST OCTET
NOTATION (BINARY)
/24 0000 0000
/25 1000 0000
Appendix C IP Addresses and Subnetting
3RD
4TH OCTET
OCTET
00000000 00000000
00000000 00000000
11111111 00000000
11111111 11111000
MAXIMUM NUMBER OF HOSTS
24
2 – 2 16777214
16
2 – 2 65534
8
2 – 2 254
3
2 – 2 6
LAST OCTET
(DECIMAL)
0
128
DECIMAL
255.0.0.0
255.255.0.0
255.255.255.0
255.255.255.248
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