ZyXEL ES-1124 User Guide - Page 33
Notation, Network Size
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Appendix B IP Addresses and Subnetting 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 6 Subnet Masks BINARY 1ST OCTET 2ND OCTET 3RD OCTET DECIMAL 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 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 7 Maximum Host Numbers SUBNET MASK 8 bits 255.0.0.0 HOST ID SIZE 24 bits 224 - 2 MAXIMUM NUMBER OF HOSTS 16777214 16 bits 255.255.0.0 16 bits 216 - 2 65534 24 bits 255.255.255.0 8 bits 28 - 2 254 29 bits 255.255.255.248 3 bits 23 - 2 6 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 8 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 ES-1124 User's Guide 33