10.10.10.1 - Subnet for directed broadcast:.For example, in an IPV4 broadcast scheme, the following subnets can be configured: When the 31-bit subnet mask address is configured on a point-to-point link, using network addresses for broadcast purposes is not allowed. IP-directed broadcast CLI configuration at the global level, or the per interface level, is not applicable on interfaces configured with a 31-bit subnet mask IP address. The largest Class A networks use a subnet mask of 255.0. This is the default subnet mask used by Class B networks and provides up to 65,536 IP addresses (256 x 256). Therefore, directed broadcast on a point-to-point interface is eliminated. 255.0 allows for close to 256 unique hosts within the network (since not all 256 IP addresses can be used). The two IP addresses are interpreted as host addresses, and do not require broadcast support because any packet that is transmitted by one host is always received by the other host at the receiving end. In contrast, a 31-bit subnet mask uses only two IP addresses: all zero bits and all one bits in the host portion of the IP address. With IPv4, four IP addresses with a 30-bit subnet mask are allocated on point-to-point networks. Support for an IPv4 address with a 31-bit subnet mask is described in RFC 3021. Network administrators have to determine how best to construct the network: more subnets and fewer hosts or more hosts. To conserve IPv4 address space, a 31-bit subnet mask can be assigned to point-to-point networks. Class C Subnet Masks The last 8 bits of the mask identify subnets and hosts. They append a suffix of the form /n to the host’s IP address to indicate the subnet mask, in which n equals the number of binary 1s in the subnet mask. Some TCP/IP configuration programs, such as those for Ascend routers, use a different notation for specifying subnet masks. The table also shows how these subnet masks would partition an IP address such as w.x.y.z into a network ID and a host ID portion.ĭefault Subnet Masks for IP Addresses Class Default Subnet Mask Network ID Host ID A 255.0.0.0 w x.y.z B 255.255.0.0 w.z y.z C 255.255.255.0 w.x.y z Ascend routers The default subnet masks for IP address classes A, B, and C are shown in the following table. Custom subnet masks: Further partition the network ID into a number of separate subnets by using a process called subnetting Default subnet masks.Default subnet masks: Partition IP addresses into their network ID and host ID portions.Two types of subnet masks are used in TCP/IP networking: Since we took one bit from the host part, we are left with. Taking the logical NOT of the subnet mask and ANDing it with the host’s IP address gives the host’s Host ID: Host = 11001111 00111101 00010000 01110111 Converting these two numbers to binary and ANDing them gives the host’s Network ID: Host = 11001111 00111101 00010000 01110111 When the inverse of the subnet mask (for example, NOT mask) is logically ANDed with the IP address of the host, the result is the host ID of the host – the portion of the host’s IP address that uniquely identifies the host on its network.įor example, consider the IP address 207.61.16.119 and the subnet mask 255.255.255.0. When the subnet mask is logically ANDed with a 32-bit IP address of a TCP/IP host, the result is the network ID of the host – the portion of the host’s IP address that identifies which network the host is on. In binary notation, decimal 0 represents the octet 00000000, and decimal 255 represents 11111111.Ī subnet mask consists of 32 binary digits, the first n of which are 1s and the remaining of which are 0s. Subnet masks are represented as four-octet dotted-decimal numbers, just as IP addresses are, except that the most common values for an octet in a subnet mask are 0 and 255.

Network subnet mask table