CCNP Certification Training:
IP Version 6 Zero Compression
By Chris Bryant, CCIE #12933
The CCNP ROUTE exam 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 CCNP ROUTE exam and gain that all-important understanding of IPv6, you've got to understand these different methods of expressing an IPv6 address.
My last IPv6 tutorial discussed leading zero compression; today we'll take a look at zero compression and combine that technique with leading zero compression.
If you have consecutive fields of zeroes, they can be expressed with two colons. It doesn't matter if you have two fields or eight, you can simply type two colons and that will represent all of them. The key here is that you can only do this once in an IPv6 address. This is referred to as zero compression. Here's an example:
Original format: 1234:1234:0000:0000:0000:0000:3456:3434
Using zero compression: 1234:1234::3456:3434
Leading zeroes in any 16-bit field can be dropped as often as you like, but each block you do this with must have at least one number remaining. For example, if the block is all zeroes, you have to leave one zero when using leading zero compression.
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.
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.
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