Copyright Goodheart-Willcox Co., Inc. 254 Principles of Cybersecurity with the number 1. The unique host address is represented with 0s. Then, the original full binary address is compared to this new string place by place. If both values are 1, then the result is 1. If both values are 0, the result is 0. If one value is 1 and the other value is 0, the result is 0. The fi nal result is a string representing the network portion of the full address in binary form. Remember: 1 AND 1 = 1, while everything else equals 0. The result of the ANDing process has identifi ed the network address of the host. In this example, if the binary result is converted back to decimal, the network address of 172.16.4.0 is revealed. The 201 is the unique portion that represents this host. Quick Look 8.1.1 Decimal and CIDR Address Formats The ability to convert addresses between decimal and CIDR formats is an important skill to master. Prac- tice converting the following examples of subnet masks to their equivalent decimal or CIDR formats. 1. The CIDR format of 255.255.192.0 is _____. 2. The CIDR format of 255.255.255.224 is _____. 3. The CIDR format of 255.255.255.128 is _____. 4. Subnet mask in decimal for CIDR format of /17 is _____. 5. Subnet mask in decimal for CIDR format of /28 is _____. 6. Subnet mask in decimal for CIDR format of /20 is _____. Private, Public, and Special Addresses Another aspect of IPv4 addresses is the concept of a private and public address and some of the special addresses used. The IPv4 protocol uses an address of 127.0.0.1 to represent the local host, or home address. The local host is the device on which you are working. Therefore, regardless of the device, the host can be referenced with this address. A private address is for use on the internal network only. It cannot be used on the Internet. Private addresses are necessary in IPv4, as the pool of available IPv4 public addresses has been exhausted. In this version, if an IPv4 address starts with any of the following octets, it is considered a private address. The lowercase X in the address represents the host portion by default. Class Private Range A 10.x.x.x B 172.16.x.x–172.31.x.x C 192.168.x.x Since private addresses cannot be placed on the Internet, they must be converted to public addresses. Many private addresses can share a single public address through a device called NAT. Network Address Translation (NAT) is a network service that converts an internal private number to a number for a public network, such as the Internet. When the packet is returned, NAT fi nds the internal reference to the private address in its table and converts the public address back to a private network address. NAT uses the unique port number for each local host to create a unique entry in its table. This is how the same public address can be used for many different private addresses. Thus, NAT is often called PNAT (port-based NAT). There is no place like 127.0.0.1! FYI