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RememberThis

Take Home Message

What to remember from this class!

Wireless

Wireless is not Ethernet (CSMA/CA is not CSMA/CD). Wireless is inherently insecure: anyone in your vicinity can sniff your traffic. Wireless is inherently unreliable: signals don't always get through if something happens to the antenna, if there's interference from a microwave or a cordless phone or a concrete wall, and so on.

Necessary Information

You must have these three pieces of information configured: a unique IP address, a reasonable subnet mask, and a correct local default router. DNS is very convenient (not technically necessary, but for practical use it is).

Troubleshooting

1. check likely failure modes first (ok, first you'll need experience or a guess)
   • what changed recently?
   • what's new?
2. check things that are very easy or very cheap to check
   • replace the cable with a known good cable
   • please mutilate bad cables to save future headaches!
3. trace the power
   • is it plugged in and turned on, power-wise?
4. trace the signal
   • is it plugged in and sending, signal-wise?
5. ask a question with a known answer
   • can I ping it?
   • don't assume anything, you might find a simple solution!
6. contact your ISP (don't suffer in silence)

Troubleshooting Software

• hardware: replace cables, maybe NIC
• ifconfig/ipconfig, netstat
• ping, traceroute/tracert, mtr, NDT
• nmap
• arp, tcpdump/wireshark

DNS Troubleshooting

• host, dig, nslookup, whois
• try another server as primary!

VLAN Facts of Life

1. 802.1Q adds 4 bytes to Ethernet packets between the Layer 2 (Ethernet) header and the Layer 3 leader, increasing the maximum packet size from 1518 to 1522 bytes; 3 bits are for the 802.1p Ethernet class of service and 12 bits are for the 802.1Q VLAN ID.
2. A VLAN, by definition, provides broadcast containment. Broadcasts (Windows browser elections, Bonjour) are contained within the VLAN.
3. Your IP address determines your IP subnet (of course). Your IP subnet uniquely determines your 802.1Q VLAN. Each VLAN can have several subnets, but each subnet has one and only one VLAN. If the VLAN on your switch port doesn't match your IP address subnet, you can't communicate on the network (except at Layer 2, but the user reports no network connectivity).
4. For traffic to travel between VLANs, it must go to the router port defined for that VLAN (your gateway). In the simplest case, it then goes through the routing engine and out the router port for the destination VLAN. (It could also go from the routing engine on one router to the routing engine on another router, and then out the router port for the destination VLAN.) Each router knows what subnets match the VLANs it manages, and it knows what router to send packets for the VLANs it doesn't manage.