Time for a Pop Quiz by James Hanback You have less time than one second. When you look at the following string of characters, shout out the first word that comes to mind.
192.168.51.50 Time’s up. It’s an Open Shortest Path First area ID. It is equivalent to the decimal value of 3232248626 in OSPF area ID.
Were you right? No? Did everyone around you jump out of their chairs or back away from you as you shouted IP ADDRESS? If you correctly guessed that 192.168.51.50 was an IP address, I’m sure I can give credit to you. However, it might not be enough credit to calm the jangled nerves in your family or coworkers if you were stupid enough to shout your answer out.
You are correct in that 192.168.51.50 represents a 32-bit value in the dotted decimal format. This is the format network administrators most familiar with as an IP address. The dotted decimal value at 192.168.51.50 is exactly what you would expect from an IP version 4 address (IPv4). It falls within the Class C range that is reserved for private networks.
An IP address is not an OSPF ID. An OSPF area ID is not an IP address. Each 32-bit address space serves a different purpose. IP addresses enable devices to communicate at Layer 3 (or the Network layer) of the Open Systems Interconnection model. OSPF areas are used to logically group OSPF routers and segregate those groups within an autonomous systems (AS). An area ID is used to identify the group or area to which a network interface or network belongs. OSPF uses area IDs in order to organize a topology, and to determine which neighbor routers it should send packets to to build the link-state database. The LSDB is a snapshot of the topology in the area. It is stored on every router within the area.
You might be thinking, “Wait just one minute.” “I have been studying for my CCNA certification, and I’ve been configuring all my practice OSPF networks areas using decimal values. I have configured the OSPF network command with 0 so many times it’s almost punctuation. What am I doing wrong?
Absolutely not.
It is normal for a novice network administrator to want to set up an OSPF area ID using a decimal value. Cisco IOS allows you to set up an area ID in two ways. You can either use a decimal value between 0 and 4294967295, or a dotted decimal value within the range of 0.0.0.0 through255.255.255.255.255.255. The values of 0.0.0.0 through 255.255.255.255.255, are valid values for an OSPF ID. The help system on a Cisco router can be used to verify it.
RouterA(config-router)#network 198.51.100.0 0.0.0.255 area ? 0-4294967295> The OSPF area ID is a decimal value A.B.C.D in IP address format
I know. I know. Cisco’s help refers the dotted decimal format to be the IP address format. This is because most people associate the dotted decimal format with IP address. As you probably proved by your answer to my pop quiz. It is important to keep the two addresses spaces separate in your mind. The area ID with the dotted decimal OSPF number has nothing to do either with the IP addresses you have assigned to interfaces or with the advertised IP networks.
The decimal range of possible area IDs’ last value, 429496729595, may at first glance seem like a strange or random way to end a range. It doesn’t feel as satisfying than a range of 0 through 1024 or 4096 or 32768 or 65535. However, if you convert that decimal value to its 32-bit binary equivalent, you get exactly 32 bits, or 11111111111111111111111111111
