add cidr to glossary (#17550)

* add CIDR

* modify cidr definition

* Update content/en/docs/reference/glossary/cidr.md

Co-Authored-By: Chris <6834943+christophercarney@users.noreply.github.com>

* Update content/en/docs/reference/glossary/cidr.md

Co-Authored-By: Chris <6834943+christophercarney@users.noreply.github.com>

content/en/docs/reference/glossary/cidr.md

@@ -0,0 +1,18 @@
+---
+title: CIDR
+id: cidr
+date: 2019-11-12
+full_link: 
+short_description: >
+  CIDR is a notation for describing blocks of IP addresses and is used heavily in various networking configurations.
+
+aka:
+tags:
+- networking
+---
+CIDR (Classless Inter-Domain Routing) is a notation for describing blocks of IP addresses and is used heavily in various networking configurations.
+
+<!--more-->
+
+In the context of Kubernetes, each {{< glossary_tooltip text="Node" term_id="node" >}} is assigned a range of IP addresses through the start address and a subnet mask using CIDR. This allows Nodes to assign each {{< glossary_tooltip text="Pod" term_id="pod" >}} a unique IP address. Although originally a concept for IPv4, CIDR has also been expanded to include IPv6. 
+