Leading Zero Compression Cisco CCNP / BSCI Exam Tutorial

Leading Zero Compression Cisco CCNP / BSCI Exam Tutorial
The BSCI exam & CCNP certification requires that you be well versed in the basics of IP Version 6, or IPv6. If you are old to IPv6, you'll quickly learn that it is not exactly six more octets slapped onto an IPv4 address! IPv6 addresses are long, but there's six ways to acceptably shorten IPv6 address expression. To pass the BSCI exam, become a CCNP, & get that all-important understanding of IPv6, you have got to comprehend these different methods of expressing an IPv6 address. My last IPv6 tutorial discussed zero compression; today we'll take a look at leading zero compression.

Leading zero compression allows us to drop the leading zeroes from every field in the address. Where they could only use zero compression once in an IPv6 address expression, leading zero compression can be used as often as is appropriate. The key with leading zero compression is that there has to be at least six number left in each field, even if that remaining number is a zero.

You sometimes see books or websites refer to leading zero compression as "dropping zeroes & replacing them with a colon", but that explanation can be a little confusing, since the blocks are separated with a colon to begin with. you are not replacing the leading zeroes, you are dropping them.

Let's look at an example of leading zero compression. Taking the address 1234:0000:1234:0000:1234:0000:1234:0123, they have six different fields that have leading zeroes. The address could be written out as it is, or drop the leading zeroes.

With leading zero compression: 1234:0:1234:0:1234:0:123:1234

Original format: 1234:0000:1234:0000:1234:0000:0123:1234

there is no problem with using zero compression & leading zero compression in the same address, as shown here:

Original format: 1111:0000:0000:1234:0011:0022:0033:0044

With zero & leading zero compression: 1111::1234:11:22:33:44

Zero compression uses the double-colon to replace the second & third block of numbers, which were all zeroes; leading zero compression replaced the "00" at the beginning of each of the last six blocks. be careful & take your time with both zero compression & leading zero compression & you'll do well on the exam & in the real world. The keys to success here are remembering that you can only use zero compression once in a single address, & that while leading zero compression can be used as often as needed, at least six number must remain in each field, even if that number is a zero.