224.0.0.0 - 239.255.255.255 - Class D
Must not be a source address, only a destination address
Not all routers talk multicast. For multicast to work between host a and host b all routers between them must understand multicast and be using the same multicast routing protocol.
IGMP V2 - protocol hosts use to tell their last hop (or first hop) router that they want to receive data for a specific multicast group (IE multicast address)
RPF - Reverse path forwarding
When a router receives a multicast packet it looks at the source address. It does a Reverse Path Forwarding check. It will check to see if the route it has in its routing table to get *to* the source address of the packet would be going out the same interface the packet was received on. It will only accept packets to be forwarded on the interface that you would use to reach the source. It will then forward the packet to all other interfaces according to its multicast routing protocol.
Multicast routing protocols routers use to determine which multicast packets go where are:
DVMRP - older, not used much any more. Flood data then prune back from sites that are not listening for a specific group.
Senders send data. If a router doesn't have any registered listeners on an interface it doesn't send the data out. If it has no registered listeners on *any* interface it sends a PRUNE message to its upstream router. Upstream routers periodically send data anyway (after the prune times out) to see if any new listeners are out there...
This protocol doesn't scale well to the entire Internet.
PIM Sparse Mode - newer, used most places today. Last hop routers contact rendezvous points (RP) to indicate they want to receive data for a group (send a JOIN to the RP). Senders send data to a RP which forwards it to all listeners.
Once a last hop router knows of a source, it can JOIN directly to the first hop router of the source.
Rendezvous points let each other know of sources via MSDP protocol (multicast source discovery protocol)
On Ethernet networks the layer 3 multicast packet is put in a multicast Ethernet frame as such:
01-00-5e-xx-xx-xx xx-xx-xx = last 24 bits of multicast IP address 224.16.32.64 would map to 01-00-5e-10-20-40 225.16.32.64 would also map to the same Ethernet address: 01-00-5e-10-20-40
FHSS - Frequency Hopping Spread Spectrum Signal jumps between frequencies in a band DSSS - Direct Sequence Spread Spectrum signal encoded across a number of frequencies in a band 2.4Ghz 2 MBPS 802.11 FHSS 2.4Ghz 2 MBPS 802.11 DSSS 2.4Ghz 11 MBPS 802.11b DSSS == WiFi 5.0Ghz 54Mbps 802.11a 2.4Ghz 54Mbps 802.11g 2.4 and 5Ghz upto 600Mbps 802.11n 5.0Ghz upto 3.5Gbps 802.11ac 802.11n can use multiple antennas and both 5/2.4 Ghz bands to get higher data rates. 802.11ac using 5Ghz bands and multiple antennas can get even higher data rates. CSMA/CA Carrier Sense Multiple Access with Collision Avoidance. Receiver must ACK reception Transmitter may or may not use RTS/CTS to "reserve" time. Usually based on size of packet. If you want to send larger packets you would use RTS/CTS Two communications methods ADHOC Infrastructure ADHOC is point to point between end stations Infrastructure uses an Access Point. All stations talk to the AP which relays between them Wireless Ethernet addresses are 48 bits just like wired Ethernet addresses. MAC header can have up to 4 addresses Source Dest Receiver Transmitter Security Modern wireless uses WPA2 (802.11i) to provide security between endpoints. Older security (WEP/WPA) not considered strong at all.