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CompTIA A+ Certification All-in-One Exam Guide

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First, leading zeros can be dropped from any group, so 00cf becomes cf

and 0000 becomes 0. Let’s rewrite the previous IPv6 address using this

shortening method:

2001:0:0:3210:800:200c:cf:1234

Second, you can remove one or more consecutive groups of all zeros,

leaving the two colons together. For example, using the :: rule, you can write

the previous IPv6 address as

2001::3210:800:200c:cf:1234

You can remove any number of consecutive groups of zeros to leave a

double colon, but you can only use this trick once in an IPv6 address.

Take a look at this IPv6 address:

fe80:0000:0000:0000:00cf:0000:ba98:1234

Using the double-colon rule, you can reduce four groups of zeros; three of

them follow the fedc and the fourth comes after 00cf. Because of the “only

use once” stipulation, the best and shortest option is to convert the address to

this:

fe80::cf:0:ba98:1234

You may not use a second :: to represent the fourth groups of zeros—only

one :: is allowed per address! This rule exists for a good reason. If more than

one :: was used, how could you tell how many groups of zeros were in each

group? Answer: you couldn’t.

Here’s an example of a very special IPv6 address that takes full advantage

of the double-colon rule, the IPv6 loopback address:

::1

Without using the double-colon nomenclature, this IPv6 address would

look like this:

0000:0000:0000:0000:0000:0000:0000:0001

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