10.5 Reserved Address Space 10.5.1 Purposes for network IDs and broadcast addresses Instructor Note The purpose of this target indicator is to introduce the concept of specially reserved IP addresses. Have the students work out the basic network numbers for all three classes of IP address. For example, for a class A address 99.0.0.0 would be a reserved network number and 99.255.255.255 would be a broadcast number. For a class B address 156.1.0.0 would be a reserved network "wire" number and 156.1.255.255 would be a broadcast number. For a class C address 203.1.17.0 would be a reserved network number and 203.1.17.255 would be a broadcast number. Also be forewarned that once subnetworks are created, the reserved network numbers and broadcast numbers become less obvious and require more work to compute. If your computer wanted to communicate with all of the devices on a network, it would be quite unmanageable to write out the IP address for each device. You might try two hyphenated addresses, indicating that you are referring to all devices within a range of numbers, but that, too, would be quite unmanageable. There is, however, a shorter method. An IP address that ends with binary 0s in all host bits is reserved for the network address (sometimes called the wire address). Therefore, as a Class A network example, 113.0.0.0 is the IP address of the network containing the host 113.1.2.3. A router uses a network's IP address when it forwards data on the Internet. As a Class B network example, the IP address 176.10.0.0 is a network address. The decimal numbers that fill the first two octets in a Class B network address are assigned and are network numbers. The last two octets contain 0s, because those 16 bits are for host numbers, and are used for devices that are attached to the network. The IP address in the example (176.10.0.0) is reserved for the network address. It will never be used as an address for any device that is attached to it.If you wanted to send data to all of the devices on a network, you would need to use a broadcast address. A broadcast occurs when a source sends out data to all devices on a network. To ensure that all of the devices on the network pay attention to the broadcast, the sender must use a destination IP address that all of them can recognize and will pick up. Broadcast IP addresses end with binary 1s in the entire host part of the address (the host field). For the network in the example (176.10.0.0) , where the last 16 bits make up the host field (or host part of the address), the broadcast that would be sent out to all devices on that network would include a destination address of 176.10.255.255 (since 255 is the decimal value of an octet containing 11111111). Web Links IP Address Subnetting Tutorial 10.5 Reserved Address Space 10.5.3 Network ID analogy Instructor Note The purpose of this target indicator is to use the postal analogy for networking. Both the postal system and the Internet use routing, the routing codes for the postal system are analogous to the routing network id numbers used on the Internet. ZIP Codes and networks IDs are quite similar in how they work. A ZIP Code enables the postal system to direct your mail to your local post office, and to your neighborhood. From there, the street address directs the carrier to the proper destination. A network ID enables a router to put a packet onto the appropriate network segment, while the host ID helps the router address the Layer 2 frame (encapsulating the packet) to the specific host on that network. 10.5 Reserved Address Space 10.5.4 Broadcast address analogy Instructor Note The purpose of this target indicator is to use the postal analogy for networking. Both the postal system and internetworks use a form of "collective" addressing. In the postal system, a bulk mailing goes to everyone with a particular postal code (typically a geographical region). In internetworks, a broadcast goes to every host with a particular network id number (typically a region of a logical network topology). A broadcast address is an address that has all 1s in the host field. When you send a broadcast packet on a network, all devices on the network notice it. For example, on a network with an ID of 176.10.0.0, a broadcast that would reach all hosts would have the address 176.10.255.255. A broadcast address is quite similar to a bulk postal mailing. The ZIP Code directs the mail to the appropriate area, and the broadcast address of "Current Resident" further directs the mail to every address. An IP broadcast address uses the same concept. The network number designates the segment, and the rest of the address tells every IP host in that network that this is a broadcast message, and that the device needs to pay attention to the message. All devices on a network recognize their own host IP address as well as the broadcast address for their network.
