Where the next subnet starts

The calculator prints one block. Overlap with yesterday's VLAN is something you check by reading both ranges.

An IPv4 prefix is a count of fixed bits. A /28 has 16 addresses. If the network address is 10.1.1.0/28, the block runs through 10.1.1.15. The next /28 on that boundary starts at 10.1.1.16. Putting a second range on 10.1.1.8/28 does not create a neighbor. It overlaps, because .8 through .15 are already inside the first block.

The usable host range is smaller than the block when the first and last addresses are reserved for network and broadcast. A point-to-point link that you numbered with those reserved addresses will not match the host range the calculator prints. Read the line that says host range, not only the prefix length.

IPv6 prefixes work the same way with a larger space. The next subnet still starts on a bit boundary. Typing a prefix that does not sit on its own boundary makes the network address change to the boundary below the address you typed. Compare the network address in the result with the address you entered. If they differ, you did not start on a boundary.

The calculator will not remember a VLAN you planned on another day. Write both ranges down and look for a host that appears in each. The tool is the subnet calculator. The longer notes are in the subnet guide.