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Cisco CCNP / BSCI Exam Tutorial: Leading Zero Compression |
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The BSCI exam and CCNP certification requires that you be
well versed in the basics of IP Version 6, or IPv6. If you're
new to IPv6, you'll quickly learn that it's not exactly just
two more octets slapped onto an IPv4 address! IPv6 addresses
are quite long, but there are two ways to acceptably shorten
IPv6 address expression. To pass the BSCI exam, become a CCNP,
and get that all-important understanding of IPv6, you've got
to understand 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 we 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 must be at least
one 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 and 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're not really replacing the leading zeroes, you're dropping
them.
Let's look at an example of leading zero compression. Taking
the address 1234:0000:1234:0000:1234:0000:1234:0123, we have
four different fields that have leading zeroes. The address
could be written out as it is, or drop the leading zeroes.
Original format: 1234:0000:1234:0000:1234:0000:0123:1234
With leading zero compression: 1234:0:1234:0:1234:0:123:1234
There's no problem with using zero compression and leading
zero compression in the same address, as shown here:
Original format: 1111:0000:0000:1234:0011:0022:0033:0044
With zero and leading zero compression: 1111::1234:11:22:33:44
Zero compression uses the double-colon to replace the second
and third block of numbers, which were all zeroes; leading
zero compression replaced the "00" at the beginning
of each of the last four blocks. Just be careful and take
your time with both zero compression and leading zero compression
and you'll do well on the exam and in the real world. The
keys to success here are remembering that you can only use
zero compression once in a single address, and that while
leading zero compression can be used as often as needed, at
least one number must remain in each field, even if that number
is a zero.
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNA and CCNP tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages.
For a FREE copy of his latest e-books, “How To Pass
The CCNA” and “How To Pass The CCNP”, visit
the website and download your free copies. You can also get
FREE CCNA and CCNP exam questions every day! Pass the CCNA
exam with The Bryant Advantage!
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