Maximum Number of Hosts per Network

One hard limit that used to apply is the 1024 device limit in a collision domain. But this really doesn't apply any more since we mostly use switches(bridges) between everything.

Ethernet: The Definitive Guide 3.6 Collision Domain

On a given Ethernet composed of multiple segments connected with repeaters, all of the stations are involved in the same collision domain. The collision algorithm is limited to 1024 distinct backoff times. Therefore, the maximum number of stations allowed in the standard for a multi-segment LAN linked with repeaters is 1024. However, that doesn't limit your site to 1024 stations, because Ethernets can be connected together with packet switching devices such as switching hubs or routers.

There is another limit which won't set a maximum number of hosts, but will set a practical limit to how many switches you can have in your network. In a switched/bridge network spanning tree is used to prevent loops, spanning tree has a maximum limit of 7 hops. This means you will be physically limited by how you arrange your switches. Ignoring this limit brought down the hospital network for almost a week.

As a general rule of thumb, stick with /24-sized VLANs: .0 for the network, .1 for the default route, .255 for the broadcast domain.

Don't try to do any fancy splitting with any of the RFC 1918 address space for things like DMZs, 'private' networks for your cluster heart beat subnets, etc.. The only place where you may not end up using a /24 is if your ISP assigns you a /27 or some such.

Generally I found that if you go for a /23 or up, it ends up being unwieldy keeping track of 500 hosts (or more). If you go with a /25 or down, each subnet ends up being too small eventually and it's a pain to keep track of where each network begins/ends.

For whatever reason /24 tends to end up being the most management from the perspective of human beings.