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Certain IP addresses are reserved and cannot be assigned to individual devices on a network. These reserved addresses include a network address, which is used to identified the network itself, and a broadcast address, which is used for broadcasting packets to all of the devices on a network. This topic describes the types of reserved IP addresses and provides examples of each.
An IP address that has binary Os in all host bit positions is reserved for the network address. Therefore, as a Class A network example, 10.0.0.0 is the IP address of the network containing the host 10.1.2.3. As a Class B network example, the IP äddress172.16.O.O is a network address, while 126.96.36.199 would be a Class C network. A router uses the network IP address when it searches its IP route table for the destination network location.
The decimal numbers that fill the first two octets in a Class B network address are assigned. The last two octets contain Os because those 16 bits are for host numbers and are used for devices that are attached to the network. In the IP address 172.16.0.0, the first two octets are reserved for the network address; it is never used as an address for any device that is attached it. An example of an IP address for a device on the 172.16.0.0 network would be 172.16.16.1, In this example, 172.16 is the network address portion and 16.1 is the host address portion.
Directed Broadcast Address
To send data to all the devices on a network, a broadcast address is used. Broadcast IP addresses end with binary is in the entire host part of the address (the host field).
For the network in the example (172.16.0.0), in which the last 16 bits make up the host field host part of the address), the broadcast that would be sent out to all devices on that network would include a destination address of 172.16.255.255. The directed broadcast is capable of being routed. However, for some versions of the Cisco lOS operating system, routing directed broadcasts is not the default behavior.
Local Broadcast Address
If an IP device wants to communicate with all devices on the local network, it sets the destination address to all is (255.255.255.255) and transmits the packet. For example, hosts that do not know their network number and are asking some server for it may use this address. The local broadcast is never routed.
Local Loopback Address
A local loopback address is used to let the system send a message to itself for testing. A typical local loopback IP address is 127.0.0.1.
Autoconfiguration IP Addresses
When neither a statically nor a dynamically configured IP address is found on startup, those hosts supporting IPv4 link-local addresses (RFC 3927) will generate an address in the 169.254/16 prefix range. This address can be used only for local network connectivity and operates with many caveats, one of which is that it will not be routed. You will mostly see this address as a failure condition when a PC fails to obtain an address via DHCP.
The network portion of an IP address is also referred to as the network ID, which is important because most hosts on a network can directly communicate only with devices in the same network. If the hosts need to communicate with devices that have interfaces assigned to some other network ID, there must be a network device that can route data between the networks. This is true even when the devices share the same physical media segment.
A network ID enables a router to put a packet onto the appropriate network segment. The host ID helps the router deliver the Layer 2 frame encapsulating the packet to a specific host on the network. As a result, the IP address is mapped to the correct MAC address, which is needed by the Layer 2 process on the router to address the frame.
Each class of a network allows a fixed number of hosts. In a Class A network, the first octet is assigned to the network, leaving the last three octets to be assigned to hosts. The first host address in each network (all Os) is reserved for the actual network address, and the final host address in each network (all is) is reserved for broadcasts. The maximum number of hosts in a Class A network is 224 — 2 (subtracting the network and broadcast reserved addresses), or
In a Class B network, the first two octets are assigned to the network, leaving the final two octets to be assigned to hosts. The maximum number of hosts in a Class B network is 216 — 2, or
In a Class C network, the first three octets are assigned to the network. This leaves the final octet to be assigned to hosts, so the maximum number of hosts is 28 — 2, or 254.