Wed Sep 01, 2021 12:49 pm
Assuming that your original "block of 5 addresses" is o.o.o.0, your 1100's own address on the ISP-facing interface is o.o.o.6, and the new "block of 5 addresses" (actually, 8 addresses if you don't waste them inefficiently) is n.n.n.0/29:
ISP's hypothetical Mikrotik:
/ip address
add interface=ether-to-ChefJay address=o.o.o.1/29
/ip route
add dst-address=0.0.0.0/0 gateway=?.?.?.?
add dst-address=n.n.n.0/29 gateway=o.o.o.6
Your 1100:
/ip address
add interface=ether-to-ISP address=o.o.o.6/29
add interface=ether-to-tenant address=100.64.0.1/30
/ip route
add dst-address=0.0.0.0/0 gateway=o.o.o.1
add dst-address=n.n.n.0/29 gateway=100.64.0.2
Tenant's hypothetical Mikrotik:
/ip address
add interface=ether-to-ChefJay address=100.64.0.2/30
/ip route
add dst-address=0.0.0.0/0 gateway=100.64.0.1
The above way, you've transparently handed over all the 8 addresses to the single tenant, and it is their job how they use them.
If you want to allocate to a given tenant only several addresses from the /29, the setup at the 1100 will be the same, except that you'll use one route per each /32 address, or possibly one route with a /31 destination prefix for two adjacent addresses or a /30 one for four. But the tenants will have to handle the addresses carefully, i.e. they will not be able to handle them as subnets as 3 IP addresses are "wasted" for a subnet. That's why a /29 is perceived as a "block of 5 addresses" although all 8 can be used if things are done slightly more carefully.