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I'm having problems connecting my LAN to aDSL.

I run a Mac Mini server with a fixed IP address, and my own named, smtpd, httpd, etc. There are two other Ethernet computers, and an Airport Extreme WiFi router (configured as a bridge), running through an 8-port switch and a D-Link router/firewall. The DLink router translates my fixed IP directly to the Mac Mini, and the others are getting DHCP from my router.

Topology of desired network.

The most noticeable symptom is web pages loading slowly or incompletely, mail not getting through, and DOWN reports from external monitoring services.

If I ping my ISP's edge router (directly from my router, or from any connected computer), it is extremely intermittent. It can be 100% for tens of seconds straight, then 50% or more packet loss for tens of seconds straight.

ping -A -n -c 10 65.38.45.1
PING 65.38.45.1 (65.38.45.1): 56 data bytes
64 bytes from 65.38.45.1: icmp_seq=0 ttl=254 time=22.780 ms
64 bytes from 65.38.45.1: icmp_seq=1 ttl=254 time=23.520 ms
Request timeout for icmp_seq 2
Request timeout for icmp_seq 3
Request timeout for icmp_seq 4
64 bytes from 65.38.45.1: icmp_seq=5 ttl=254 time=23.207 ms
64 bytes from 65.38.45.1: icmp_seq=6 ttl=254 time=23.774 ms
Request timeout for icmp_seq 7
64 bytes from 65.38.45.1: icmp_seq=8 ttl=254 time=23.608 ms
64 bytes from 65.38.45.1: icmp_seq=9 ttl=254 time=24.670 ms

--- 65.38.45.1 ping statistics ---
10 packets transmitted, 6 packets received, 40.0% packet loss
round-trip min/avg/max/stddev = 22.780/23.593/24.670/0.579 ms

If I hook a computer directly to the aDSL modem via DHCP, everything works, so my ISP says, "Sorry, that's YOUR problem, NOT ours." They've had a phone company tech out to the network interface, who says everything is fine there.

Topology of zero packet loss network.

But if I plug even a second computer in (both computers relatively quiescent), via router or switch, then packets begin to get dropped.

Topology of high packet loss network.

I understand that if all my computers were doing huge downloads at the same time, I could expect packet loss. But this happens simply by plugging in a second computer that is doing no network traffic.

Here's the weird part: after having my laptop plugged directly into the aDSL modem and seeing 100% success, if I plug it into my "normal" network, things will work just fine for several minutes before packets start getting dropped again! I can run 100% ping tests on any computer on my LAN, and things work fine for several minutes.

To me, this smells of "traffic shaping," where the ISP is detecting that I have more than one computer plugged in, and then reduces bandwidth. They claim they don't do that.

I'm pretty knowledgable about networking, but this has me stumped.

I'm at my wits end. Any ideas how to debug and fix this?

UPDATE +1 to "sawdust" for suggesting I go back to basics. I paired the laptop with different combinations of other computers, and found that only when the Mac Mini Server AND the D-Link were hooked up, things went south. I took the router out, and hooked the switch directly to the modem, and things started working. Thanks, D-Link.

enter image description here

This is a configuration I thought I had tried before, but I guess not, or something else was wrong at the time.

But now I can't get my printer working. It keeps wanting to be in the 10.129 network, and other computers can't see it there. Keep working on it, I guess...

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"if I plug even a second computer in ... via router or switch..." - This doesn't seem correct. The ZOOM unit is a modem/router/firewall. So you have two routers in your setup. Where are the DHCP and NAT servers? Are there 2 servers or zero servers? Or are you comparing apples to oranges by reconfiguring the ADSL modem? If you have DHCP, NAT and a firewall properly setup, then the ISP should not be able to detect how many PCs you have. BTW the "A" is ADSL should be capitalized like the rest of the letters in the acronym. – sawdust Oct 8 '12 at 19:26
The ZOOM 5751 that my ISP provided does not appear to be a router nor firewall. It has only one ethernet port. The D-Link router is my DHCP/NAT server, doling out addresses in the 10.1.1.0/24 subnet. All other services (DNS, web, mail, MySQL, etc.) are coming from the Mac Mini, which is NOT serving DHCP nor NAT. – Jan Steinman Oct 8 '12 at 19:58
"It has only one ethernet port" - That means nothing. I've used Westell units that have full-featured router capability, and they also have a single Ethernet port. Maybe you need to log into the ZOOM and find out what features it has and how it is configured, rather than just guess. Multiple routers would be a plausible explanation for the cause of your issues. Of course you don't have to test your assumptions, and instead go off believing that the ISP is "traffic shaping" your account. – sawdust Oct 8 '12 at 20:15
Also, the Airport Extreme is configured as a bridge, and so should not be serving DHCP nor NAT, but only passing along DHCP and NAT from the D-Link router. – Jan Steinman Oct 8 '12 at 20:15
1  
Go back to the simplest setup, the ZOOM + laptop. Does the laptop get a private or public IP address? What does the "Shields Up" site, www.grc.com, report? – sawdust Oct 8 '12 at 20:25
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UPDATE +1 to "sawdust" for suggesting I go back to basics. I paired the laptop with different combinations of other computers, and found that only when the Mac Mini Server AND the D-Link were hooked up, things went south. I took the router out, and hooked the switch directly to the modem, and things started working. Thanks, D-Link.

enter image description here

This is a configuration I thought I had tried before, but I guess not, or something else was wrong at the time.

But now I can't get my printer working. It keeps wanting to be in the 10.129 network, and other computers can't see it there. I sent all my MACs to my ISP, who said they would provide dynamic IPs in the same subnet as my static IP, which should let me see the printer again.

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