Prefix List Tools
Prefix Search Tools
Asia
Autralia
Canada
China
Europe
Germany
India
Japan
South America
South Korea
United Kingdom
US
Office 365 Services
Generate Prefix List
Generate Complement Prefix List
Output format:
Summarize Prefix List

Background

This API can allow you to retrieve (programatically or using this page) to retrieve IP prefixes in Azure and for Office 365. The IP address space for Azure represents the IPs that get advertised over ExpressRoute's public peering, whereas the IPs for Office 365 do not represent necessarily all the IPs advertised over ExpressRoute's Microsoft peering but some. See more here.

Prefix list or Complement Prefix List?

Prefix list is what it is. Try and see. Complement Prefix are interesting in that the represent what's "not" the prefix itself. For instance, if you have a list with only one prefix of 10.0.0.0/8, the complement would be :
0.0.0.0/6 (covering from 0.0.0.0 to 7.255.255.255)
8.0.0.0/7 (covering from 8.0.0.0 to 9.255.255.255)
11.0.0.0/8 (covering from 11.0.0.0 to 11.255.255.255)
12.0.0.0/6 (covering from 12.0.0.0 to 15.255.255.255)

Details

Advertising 0.0.0.0/0 on the BGP sessions with the Microsoft Enterprise Edge routers achieves the forced tunneling. To ensure that not all (a.k.a. 0.0.0.0/0) traffic is sent back on premise and what is meant for the Azure Datacenter to stay in Azure, you would advertise the Complement Prefixes. That keeps the cloud in the cloud.

Disclaimer

Use at your own risks on your routers. Like any production network, you need to understand fully the impact of using these configuration lines into your production routers. The authors provide no guarantee on that code, implicit or explicit and aren't liable for any consequence, including loss of productivity, profits, etc.