First there were two wires.
Then four and finally eight.
From the beginning the power transfer was also an option.
Before the current standard there were also cables with less than four pairs.
Those cables were usually targeting multiple connection methods.
Current standard A means that old system is used.
There pairs 1 and 2 are used for data, so for that purpose pairs 3 and 4 are irrelevant.
Later early version of power was added and pairs 3 and 4 were used, since power is unidirectional those pairs were not crossed, obvious back then.
Next phase was early phase of current PoE.
But since old style power transfer devices were still all around only pairs 1 and 2 were used.
Later B standard faster fours pair system became dominant.
But it still must recognize the old system.
So for A system data, Halcyon's bicture, you cross pairs 1 and 2, pairs 3 and 4 you can leave out, if you like.
For B system you also cross pairs 3 and 4, but normally B system is automatic.
IP addresses must be in the same network.
IPv4 mask 255.255.255.0 is a 32bit number, there set(1) bit means network.
If one address is 192.168.0.100 and mask is as above three left octets define a network.
So all 192.168.0.nnn addresses are in that network and can communicate.
Leave few last and first numbers for special purposes.
DHCP is a protocol where server is allocating addresses.
If that kind of thing is part of the network don't use its address range for manually configured devices.
Usually server is allocating addresses from the beginning of its range but don't count on it.
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