cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
Announcements
Join Customer Connection to register!
2225
Views
0
Helpful
3
Replies
black_adder
Beginner

Definition of class E IP addresses used by Cisco

According to RFC documents, Douglas Comer's book "Computer networks and internets" and English wikipedia, class E IP addresses are those with leading bits 1111 (values of remaining bits are arbitrary). On the other hand,  Cisco's documents and one of IBM Redbooks - "TCP/IP Tutorial and Technical Overview" - define class E addresses as those with leading bits 11110 (fifth highest bit has a fixed value of 0). How to explain this difference? To make matters worse, the definition of class E in this document:

http://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/13788-3.pdf

is self contradictory - according to the left side of the diagram (Figure 1) class E is limited to addresses starting with bits 11110 (not 1111), but at the same time the corresponding right side of the diagram implies that the highest octet of class E address can be equal to 254 which is impossible since the highest possible value of the highest octet should be binary 11110111 (decimal 247). I'm confused. Can you please explain it and, if there's indeed an error in your materials, can you fix it?

3 REPLIES 3
e.ciollaro
Enthusiast

I don't know why cisco and IBM say 11110 but RFC 5735 is very clear:

  240.0.0.0/4 - This block, formerly known as the Class E address
   space, is reserved for future use; see [RFC1112], Section 4.

So it's 1111xxxx

 

Bye,

enrico

That's what I thought. Honestly, it's hard to believe such a fundamental error could remain unnoticed for years by so many people. Am I really the first one who spotted it? After all, Cisco is considered one of the most reputable companies in the world as far as computer networks are concerned, so I'd expect high quality training materials from them. Out of curiosity, I'm still waiting for someone from Cisco to explain why their definition differs from the one in RFC.

I sent in errata in the past with Cisco Official Guides and stuff, nothing changed. If you download errata for Cisco books, you will be surprised how misleading sometimes they can be for a beginner.