31 Days Before Your CCNA Exam

Day 21 111Note Today your ISP assigns you a block of addresses that has no relationship to the originalclasses. Based on your needs, you could be assigned one or more addresses. But withthe use of NAT and private addresses inside your network, rarely will you need more than ahandful of public IP addresses.In a classful addressing scheme, devices that operate at Layer 3 can determine the address class ofan IP address from the format of the first few bits in the first octet. Initially, this was important sothat a networking device could apply the default subnet mask for the address and determine thehost address. Table 21-1 summarizes how addresses are divided into classes, the default subnetmask, the number of networks per class, and the number of hosts per classful network address.Table 21-1IP Address ClassesAddress First First Octet Bits Network (N) Default Number ofClass Octet (Highlighted and Host (H) Subnet Mask PossibleRange Bits Do Not Portions of (Decimal Networks(Decimal) Change) Address and Binary) and HostsPer NetworkA 1–127 00000000–01111111 N.H.H.H 255.0.0.0 2 7 or 12711111111.00000000.00000000.00000000 networks2 24 –2 or16,777,214 hostsper networkB 128–191 10000000–10111111 N.N.H.H 255.255.0.0 2 14 or 16,38411111111.11111111.00000000.00000000 networks2 16 –2 or 65,534hosts per networkC 192–223 11000000–11011111 N.N.N.H 255.255.255.0 2 21 or11111111.11111111.11111111.00000000 2,097,152 networks2 8 –2 or 254 hostsper networkD 224–239 11100000–11101111 Not used for host addressingE 240–255 11110000–11111111 Not used for host addressingIn the last column, the “minus 2” for hosts per network is to account for the reserved network andbroadcast addresses for each network. These two addresses cannot be assigned to hosts.Note We are not reviewing the process of converting between binary and decimal. At thispoint in your studies, you should be very comfortable moving between the two numberingsystems. If not, take some time to practice this necessary skill. Refer to the “StudyResources” section. You can also search the Internet for binary conversion tricks, tips, andgames to help you practice.Purpose of the Subnet MaskSubnet masks are always a series of one bits followed by a series of zero bits. The boundary wherethe series changes from ones to zeros is the boundary between network and host. This is how adevice that operates at Layer 3 determines the network address for a packet—by finding the bit