You are partly correct. The IP is the node who first told Blockchain.info (assuming that's what this is using) about the transaction. The "entry point" to the n

You are partly correct. The IP is the node who first told Blockchain.info (assuming that's what this is using) about the transaction. The "entry point" to the network would be when the person sending first transmits it. There is no way to be sure where a transaction came from, but if you have data about how long it took to reach parts of the network, you can make an educated guess (which becomes more accurate if you have more connections/nodes). Some people specifically add Blockchain.info to their peer list, but that is bad for anonymity since all your transactions will then list your real IP in the "relayed by" field.