IPv6 addresses are 128 bits, written as eight groups (hextets) of four hexadecimal digits separated by colons, such as 2001:0db8:0000:0000:0000:0000:0000:0001. Two rules shorten them. Leading zeros in any group can be dropped, so 0db8 becomes db8 and 0010 becomes 10. One run of consecutive all-zero groups can be replaced by a double colon, so the address above becomes 2001:db8::1. You may use the double colon only once, otherwise nobody could tell how many zero groups each one stands for. When two runs of zeros exist, the convention is to compress the longest one.
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