When pentesting a network, call it ExampleNet, the network range sometimes exceeds 256 hosts, such as the range x.y.56.0-x.y.63.255. Does this mean that a compromised host on x.y.56.z could be used to leverage a MITM attack against a range such as x.y.62.z, considering they both share the same ethernet?
It generally depends on network setup. Some commercial switches can separate the network out into different subnets, with special routing tables between them. That way you'll have a single physical switch connecting all the machines, but you'll still have several subnets.
They may also be using technologies like ipsec to encrypt and authenticate network communications, which can make a MITM attack difficult, if not impossible.
But, if it's all on the same switch, the switch isn't doing anything "clever", and they're not using a protection technology such as ipsec, then you should be able to perform a MITM attack using a single comprimised host.