Subnets

A subnet is an IP-address block inside a network. VMs and load balancers attach to subnets, not directly to networks — a network is just a container that one or more subnets live inside.

CIDR sizing

You pick the CIDR at creation time. Typical sizes:

Every subnet reserves: the network address (first), the broadcast (last), the gateway (typically .1), and our DHCP server (typically .2). So a /24 subnet has 256 raw addresses but only 251 usable for VMs.

You can resize a subnet larger (/27 → /25) online without disruption. You cannot shrink a subnet without recreating it; existing VM IPs would have to be renumbered.

DHCP, SLAAC, and gateway behavior

• IPv4 subnets use DHCP by default. Each VM gets a fixed address from the subnet's available range. You can pre-allocate specific IPs via the ports endpoint when you need a stable address (e.g. for an LB backend in DNS).
• IPv6 subnets use SLAAC by default. Each VM auto-generates an address from the subnet's /64 prefix + its MAC address. DHCPv6 is available as an opt-in for environments that require deterministic addressing.
• Gateway is set per subnet. For tenant subnets that need internet access, the gateway points at the router attached to the subnet; for the external network's auto-created subnet, the gateway points at our edge.

Multiple subnets per network

You can attach multiple subnets to one network. A VM connected to that network can be reachable from every subnet inside it (subject to security groups). This is useful for IPv4 + IPv6 dual-stack — one v4 subnet + one v6 subnet on the same network gives every VM both stacks.

What's next

• Networks — the parent of subnets.
• Routers — what to attach a tenant subnet to so it can reach the internet.
• Virtual machines — VMs attach to subnets via ports.