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Misconceptions about Networking*

Time to fess-up!... 'at some point' you thought you knew something, and it ended up not being correct, or not entirely correct due to a misconception about the subject.

Let's build a good list of popular misconceptions novice AND even some seasoned IT administrators have, explicitly about Networking. My hope is to build a very useful brain-dump to serve as a good resource for the members of this community.

I'll start with an extremely obvious example (items with the most votes up will be on top):

• All addresses beginning with 169 come from the APIPA failover system

  Only 169.254.0.0/16 is reserved for the APIPA assignation when the OS can't find an assigned address for a network interface (read:rfc3927).

*****Not to be mistaken with "Mistakes made by sysadmins"

Myth: Allowing ICMP is insecure.

This one is a pet peeve of mine, and it's widespread enough to cause significant problems on the Internet. Aside from the handy diagnostics we all know and love, there's Path MTU Discovery and other things that break when ICMP is blocked.

Some people have religious believes about allowed and not allowed IP addresses. Yesterday I saw in one of the answers here that 'IP addresses ending in .0 or .255 are invalid' which is plain wrong.

Others still think that we have A, B, C - sized subnets only, while CIDR was ruling the world for quite a while.

Some claim that disabling ICMP responses will make my workstation invisible in its LAN segment, which is not true. You can still send ARP requests and in most cases machine will send ARP response although it has IP level firewall running.

Others say private subnets - 192.168.0.0/16 or 10.0.0.0/8 are 'not routable' - which is plain wrong again.

People get really surprised when they learn how upload saturation affects their download speeds. This very much depends on the queuing algorithms on both ends of bottleneck, but in case of typical ADSL connections upload can significantly affect download.

On the funny side: some still think that "The Internet is a Series of Tubes".

All Internet connections are created equal aka download speed is the only thing that matters

"I just found out what a T1 is, don't you know that my Comcast cable at home is 6x faster than our connection at work? Why don't we have that here?"

It doesn't come up any more, but it got to the point where I'd rather swallow staples than try to explain an SLA to yet another marketing guy. (OK, I had the same question when I was a workstation support scrub, I admit it!)

James Gosling cites Peter Deutsch with credit for the eight fallacies of distributed computing:

Essentially everyone, when they first build a distributed application, makes the following eight assumptions. All prove to be false in the long run and all cause big trouble and painful learning experiences.

1. The network is reliable
2. Latency is zero
3. Bandwidth is infinite
4. The network is secure
5. Topology doesn't change
6. There is one administrator
7. Transport cost is zero
8. The network is homogeneous

I have these on the wall of my cube facing the hallway. Sometimes I feel that I trip over more than one of these per day.