The subnet calculator helps you design networks by taking an IP address and network mask and returning information such as network, broadcast address, and host range.
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Assuming you start with five siblings ordered 1,2,3,4,5
If you want to insert a new one after 2, just increase the number for all siblings above 2 by one, starting with the highest number (5) and going down to 3 (2+1).
5->6
4->5
3->4
The new ordering numbers are now 1,2,4,5,6
Add the new sibling with number 3 and you're done.