ZyXEL Communications SBG3500-N000 User Manual page 358

Appendix B IP Addresses and Subnetting
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.
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 129 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 130 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 255.255.255.24
8
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.
358
BINARY
1ST 2ND
OCTET OCTET
11111111 00000000
11111111 11111111
11111111 11111111
11111111 11111111
HOST ID SIZE
24 bits
16 bits
8 bits
3 bits
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
DECIMAL
255.0.0.0
255.255.0.0
255.255.255.0
255.255.255.248
SBG3500-N000 User's Guide