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In addition to extending the number of addresses available, subnet addressing provides other benefits. Subnet addressing allows a network manager to construct an address scheme for the network by using different subnets for other geographical locations in the network or for other departments in the organization. Although the preceding example uses the entire third octet for a subnet address, note that you are not restricted to octet boundaries in subnetting. To create more network numbers, you need only shift some bits from the host address to the network address. For instance, to partition a Class C network number (192.68.135.0) into two, you shift one bit from the host address to the network address. The new netmask (or subnet mask) is 255.255.255.128. The first subnet has network number 192.68.135.0 with hosts 192.68.135.1 to 129.68.135.126, and the second subnet has network number 192.68.135.128 with hosts 192.68.135.129 to 192.68.135.254. Ne twor k and Rou t in g B a s i c s Reference Guide for the Model RP114 Web Safe Router Note: The number 192.68.135.127 is not assigned because it is the broadcast address of the first subnet. The number 192.68.135.128 is not assigned because it is the network address of the second subnet. The following table lists the additional subnet mask bits in dotted-decimal notation. To use the table, write down the original class netmask and replace the 0 value octets with the dotted-decimal value of the additional subnet bits. For example, to partition your Class C network with subnet mask 255.255.255.0 into 16 subnets (4 bits), the new subnet mask becomes 255.255.255.240. Table B-1. Netmask Notation Translation Table for One Octet Number of Bits Dotted-Decimal Value 1 128 2 192 3 224 4 240 5 248 6 252 7 254 8 255 The following table displays several common netmask values in both the dotted-decimal and the masklength formats. Table B-2. Netmask Formats Dotted-Decimal Masklength 255.0.0.0 /8 255.255.0.0 /16 255.255.255.0 /24 255.255.255.128 /25 255.255.255.192 /26 255.255.255.224 /27 255.255.255.240 /28 255.255.255.248 /29 B-6 Network and Routing Basics Reference Guide for the Model RP114 Web Safe Router Table B-2. Netmask Formats 25 5 . 25 5. 2 5 5 . 2 52 / 3 0 25 5 . 25 5. 2 5 5 . 2 54 / 3 1 25 5 . 25 5. 2 5 5 . 2 55 / 3 2 NETGEAR strongly recommends that you configure all hosts on a LAN segment to use the same netmask for the following reasons: • So that hosts recognize local IP broadcast packets When a device broadcasts to its segment neighbors, it uses a destination address of the local network address with all ones for the host address. In order for this scheme to work, all devices on the segment must agree on which bits comprise the host address. • So that a local router or bridge recognizes which addresses are local and which are remote Private IP Addresses If your local network is isolated from the Internet (for example, when using NAT), you can assign any IP addresses to the hosts without problems. However, the IANA has reserved the following three blocks of IP addresses specifically for private networks: 10.0.0.0 - 10.255.255.255 172.16.0.0 - 172.31.255.255 192.168.0.0 - 192.168.255.255 NETGEAR recommends that you choose your private network number from this range. The DHCP server of the Model RP114 router is preconfigured to automatically assign private addresses. Regardless of your particular situation, do not create an arbitrary IP address; always follow the guidelines explained here. For more information about address assignment, refer to RFC 1597, Address Allocation for Private Internets, and RFC 1466, Guidelines for Management of IP Address Space. The Internet Engineering Task Force (IETF) publishes RFCs on its Web site at Ne twor k and Rou t in g B a s i c s Reference Guide for the Model RP114 Web Safe Router Single IP Address Operation Using NAT In the past, if multiple PCs on a LAN needed to access the Internet simultaneously, you had to obtain a range of IP addresses from the ISP. This type of Internet account is more costly than a single-address account typically used by a single user with a modem, rather than a router. The Model RP114 router employs an address-sharing method called Network Address Translation (NAT). This method allows several networked PCs to share an Internet account using only a single IP address, which may be statically or dynamically assigned by your ISP. The router accomplishes this address sharing by translating the internal LAN IP addresses to a single address that is globally unique on the Internet. The internal LAN IP addresses can be either private addresses or registered addresses. For more information about IP address translation, refer to RFC 1631, The IP Network Address Translator (NAT). The following figure illustrates a single IP address operation. Private IP addresses 192.168.0.2 192.168.0.3 192.168.0.4 192.168.0.5 192.168.0.1 172.21.15.105 assigned by user Internet IP addresses assigned by ISP 7786EA Figu...