docs: FIB 2.0 start

Change-Id: I87cd2eae133c9f5b9f7764a0f7a41bcc28523e4c
Signed-off-by: jdenisco <jdenisco@cisco.com>

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+.. _prefixes:
+
+Prefixes
+^^^^^^^^
+
+Some nomenclature used to describe prefixes:
+
+* 1.1.1.1 This is an address since it has no associated mask
+* 1.1.1.0/24 This is a prefix.
+* 1.1.1.1/32 This is a host prefix (the mask length is the size of the address).
+
+Prefix A is more specific than B if its mask length is longer, and less specific if
+the mask is shorter. For example, 1.1.1.0/28 is more specific than 1.1.1.0/24. A
+less specific prefix that overlaps with a more specific is the **covering** prefix.
+For example, 1.1.1.0/24 is the covering prefix for 1.1.1.0/28 and 1.1.1.0/28 is termed
+the **covered** prefix. A covering prefix is therefore always less specific than its
+covered prefixes.