ok a diagram would help explain better what you are trying to achieve. Your ISP should not be concerned or allocating IP space on your internal network. An ISP would usually give you a /30 or /29 on your WAN side of things. Internal side of things, IP allocation for vlan's is usually completed by your network person (possibly you) based on hosts / devices needed. Typically you would have multiple vlan's for security reasons, aka office traffic, management traffic etc. A /22 is a huge broadcast domain. You need to think of amount of devices on vlan, allow for some growth. If its a small site then a /24 (of the /22) is all thats required.
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