Appendix B IP Addresses and Subnetting
P-660HN-51 User’s Guide
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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 105 Subnet Masks
BINARY
DECIMAL
1ST OCTET 2ND OCTET 3RD OCTET 4TH OCTET
8-bit mask 11111111 00000000 00000000 00000000 255.0.0.0
16-bit mask 11111111 11111111 00000000 00000000 255.255.0.0
24-bit mask 11111111 11111111 11111111 00000000 255.255.255.0
29-bit mask 11111111 11111111 11111111 11111000 255.255.255.248
Table 106 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 107 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
255.255.255.224 /27 1110 0000 224
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