ZyXEL NBG6615 User Guide - Page 141
Notation, Subnetting, Subnet Mask, Host Id Size, Maximum Number Of Hosts, Alternative, Last Octet
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Appendix A IP Addresses and Subnetting As these two IP addresses cannot be used for individual hosts, calculate the maximum number of possible hosts in a network as follows: Table 52 Maximum Host Numbers SUBNET MASK HOST ID SIZE 8 bits 255.0.0.0 24 bits 16 bits 255.255.0.0 16 bits 24 bits 255.255.255.0 8 bits 29 bits 255.255.255.248 3 bits 224 - 2 216 - 2 28 - 2 23 - 2 MAXIMUM NUMBER OF HOSTS 16777214 65534 254 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 53 Alternative Subnet Mask Notation SUBNET MASK ALTERNATIVE NOTATION LAST OCTET (BINARY) 255.255.255.0 /24 0000 0000 255.255.255.128 /25 1000 0000 255.255.255.192 /26 1100 0000 255.255.255.224 /27 1110 0000 255.255.255.240 /28 1111 0000 255.255.255.248 /29 1111 1000 255.255.255.252 /30 1111 1100 LAST OCTET (DECIMAL) 0 128 192 224 240 248 252 Subnetting You can use subnetting to divide one network into multiple sub-networks. In the following example a network administrator creates two sub-networks to isolate a group of servers from the rest of the company network for security reasons. In this example, the company network address is 192.168.1.0. The first three octets of the address (192.168.1) are the network number, and the remaining octet is the host ID, allowing a maximum of 28 - 2 or 254 possible hosts. The following figure shows the company network before subnetting. NBG6615's User's Guide 141
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