Exchange whitelist sender IP Address ranges?

Where would one whitelist a customers IP address range in Exchange 2013 AND Exchange Online?

Example:
Is this done by the following?

     Add-IPAllowListEntry -IPAddress X.X.X.0/18

They asked us to update our SPF record to add include:theirserver.... which I'm not truly sure we want to do.

But at least the IP Address range would be acceptable...

Ideas? Thanks in advance
Christian HansUndecided... Asked:
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Gaurav SinghHead - Managed ServicesCommented:
Yes it is done by Add-IPAllowListEntry -IPAddress X.X.X.0/18

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