Falsehoods programmers believe about networks

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Inspired by falsehoods programmers believe about time and usernames, I thought I'd start collecting falsehoods programmers have about networks.

1. Data on the network cannot be altered.
2. Encrypted data on the network cannot be altered.
3. Data cannot be accidentally corrupted, because TCP has checksums and Ethernet has CRCs
4. If it's inside my perimeter firewall, that means I have total control over it (@armorguy)
5. If it doesn't return an error, then send() sent all the data that was asked of it.
6. Packets arrive in the order in which they were sent.
7. Segment boundaries on a TCP stream are meaningful to the application.
8. Segment boundaries on a TCP stream are not meaningful to the application.
9. If you can't ping the target, then it doesn't exist. (@jjarmoc)
10. If you can ping the target, then it does exist.
11. TCP RSTs come from end-nodes.
12. Bytes must be "swapped" from the network byte-order to the host CPU byte-order.
13. It's an internal web app -- outsiders won't be able to discover where it is (@biosshadow)
14. The DHCP address will be the same after a reboot (@shewfig)
15. The DHCP address will remain the same until the next reboot.
16. Well, it'll last a long time between changes
17. Packets/PDUs go up or down the network stack, never sideways. (@maradydd)
18. The IPv4 header is 20 bytes long starting with 0x45 (options are so rare we don't have to worry about them) (@shewfig)
19. The DHCP server and local router are the same (@schrotthaufen)

What's fun is that you can see these errors happen by monitoring packets, I started this list for programmers, but we inevitably drifted outside programmers to network administrators. It's hard to draw the line, because some misconceptions are shared by both.

1. There is no IPv6 on my network (@shewfig)
2. NAT automatically blocks all inbound attacks (@shewfig)
3. We know all the devices attached to our network at any given time (@armorguy)
4. VLANs are just as good as physical segmentation. (@jjarmoc)
5. Ok, VLANs aren't as good, but they are good enough for now.
6. We have good WIPS/monitors, so we don't have rogue access-points anywhere. (@armorguy)
7. No need to add it to the DNS; I'll remember it. (@shewfig)