Archive for RFC 1918

Addressing a Secret Agent! (no, not really)

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on November 3, 2011 by jjrinehart

Agent 004 1/2

For me, there will always be only ONE James Bond, namely Sean Connery.  While that certainly dates me a little bit (I am in my 40’s, you can stop laughing now), I just never cared for the subsequent incarnations of the role by the later actors, and I certain envy Connery’s continued appeal and longevity.  In any case, James Bond represents the consummate “black ops” agent, although the older term most of us grew up with was spy or secret agent.

That thought makes a great entry point into a particularly helpful area of network knowledge, namely, private addressing.  As we discussed earlier, IP addressing assumes the ability to globally route packets based on the source and destination addresses contained in the IPV4 header.  As with many human inventions, however, there were unexpected flaws in the Internet, and it became the victim of its own success.  Because of the vast popularity of the commercial Internet, IP address space was rapidly getting utilized, creating a threat called address exhaustion (think of it as a widespread shortage).  One of the mechanisms created to address this problem was the creation of private addressing, defined in RFC
1918 and 4193.  Private addressing operates under the legitimate assumption that every device does not need or require a globally routable address and even encouraged the use of groups of addresses that are not allowed on the Internet (we will discuss Network Address Translation in the next article).  Three ranges, each in a separate address class, were designated for private addressing, as follows:

Class Networks Range # of Addresses
CIDR Notation Network/Mask Notation
A – 16,777,216
B –
C –

In my personal experiences as a network professional, I can tell you that the range I have seen on most enterprise networks is from the range.  That makes sense, because the address space is incredibly vast and unlikely to be exhausted in just about any environment!  I have also
seen some usage in the range, particularly in consumer devices.  Cisco/Linksys devices use this range by default, and even some of the business-grade units, such as the ASA 5505, utilize this set of addresses.  The advantages are that they are well-suited for smaller environments and fairly straightforward overall.  The range is one that I have seen on a few, very limited and rare
occasions.  Honestly, I am curious about the reasons for this.

In our next discussion, we will consider the fraternal twin of private addressing, namely, Network Address Translation.

–  Joe