Policies - CIDR Notation

This article contains information on configuring Attachment Management Bypass policies in Mimecast, allowing administrators to bypass specific attachment policies, including Attachment Block on Size, Attachment Link on Size, and Attachment Management.

Classless interdomain routing (CIDR) is an efficient way of allocating IP addresses.
Across the Mimecast product stack, CIDR notation is required whenever there is an option to input IP addresses. CIDR stands for Classes Inter-Domain Routing and is an efficient way of subnetting IP addresses.

CIDR notation

CIDR notations are used to reduce the complexity of handling IP addresses and ranges. 

IP addresses written using CIDR notation use the following naming structure:

n.n.n.n/x

• • • n.n.n.n is the Octet Prefix.
    • /x is the Suffix.
      
      The suffix of an IP address defines how many IP address bits are contained within that address. 
      IPv4 addresses are made up of 4 octets, meaning there are a possible 32 bits in total. Therefore, the suffix can be between 0 and 32. The suffix demonstrates the number of IP addresses in that network. 

Suffix	Number of IP addresses
/32	1
/24	256
/16	65,536
/8	16,777,216

CIDR and Security

Within Mimecast, IP addresses are often used to secure and control access. It is important to specify a valid IP address with the correct suffix. 
Inputting the incorrect suffix may inadvertently introduce a security risk. In some circumstances, Mimecast limits IP addresses to/24.