Typically when a customer orders a leased line, we'll supply the service with a managed router and configure a linknet between the router in our datacentre and the router at your premises. Then we'll statically route your assigned IPs over this linknet. In this scenario, if your assigned IP range (for example) were 192.0.2.0/29, we would assign 192.0.2.1 to our managed router as the gateway address leaving you with 192.0.2.2 - 192.0.2.6 to assign as you desire.
For simplicity, this would be communicated to you as follows:
IPs to assign your end: 192.0.2.2 -> 192.0.2.6.
Subnet Mask: 255.255.255.248 (/29)
Gateway: 192.0.2.1
We also supply IPv6 ranges as standard and we'd assign this as per your preference.