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Here is an excerpt from Andrew S. Tanenbaum, Computer Networks, 5th edition, Chapter 5 (The Network layer), Page 455:

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My question is on the second paragraph. From what I understand, basically this part talks disadvantage of using NAT(Network Address Translation). I don't understand what second paragraph is trying to say about disadvantage of using NAT because it seems to me that the second paragraph is saying about benefit of using NAT because by using NAT, we can solve the problem of depletion of IP address.

Can anyone explain to me what the second paragraph is trying to convey?

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The practical limit to the number of hosts that can use a single NAT'd address is much less than 61,440. This is because even a modest Internet-capable computer will use many simultaneous TCP connections to hosts beyond the firewall. Each of these connections, while it is open or being negotiated open or closed, counts against that 61,440 figure. If each local host needs to reliably be able to initiate 200 external TCP connections at once, then that 61,440 value is reduced to only 307 hosts. This won't be a problem for a typical home network (yet), but a small business could easily reach this limit.

If the TCP source port size were larger, then that would offer a correspondingly larger number of address:port pairs for address translation to use, permitting many more hosts and connections from behind the firewall.

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