• traffic isolation: frames to/from ports 1-n can only reach ports 1-n can also define VLAN based on MAC addresses of endpoints, rather than switch port.

• dynamic membership: ports can be dynamically assigned among VLANs

• forwarding between VLANS: done via
  routing (just as with separate switches)
  • in practice vendors sell combined switches plus routers

VLAN Advantages

• trunk port: carries frames between VLANS defined over multiple physical
  switches

  • frames forwarded within VLAN between switches can’t be vanilla 802.1 frames
    (must carry VLAN ID info)

  • 802.1q protocol adds/removed additional header fields for frames forwarded
    between trunk ports

Trunking

• To dynamically obtain IP address from network server

• Leasing

  • IP address is not assigned forever

• Can Reuse address pool

• Supports mobile users

DHCP

How does DHCP work

DORA Phases:

1. Discover: A host broadcasts a request for an IP address on its network using a DHCP DISCOVER packet.
2. Offer: Once a DISCOVER packet is received by a DHCP server the server will allocate a free IP address and send it to the host using a DHCP OFFER packet.
3. Request: Once an OFFER packet is received by a host it will send a DHCP REQUEST packet back to the DHCP server.
4. Acknowledge: The server as a final act will send a DHCP ACKNOWLEDGE packet to the host.

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Does DHCP use TCP or UDP?

• Local Network uses a SINGLE IP Address

• How many and the ranges of IP addresses are irrelavant to ISP

• Can alter local IP addresses without notifying the world

• ISP can be changed without changing local addresses

• Local devices are protected as they cannot be seen from the outside world

Network Address Translation

• outgoing datagrams: replace(source IP address, port #) of every outgoing datagram to (NAT IP address, new port #) . . . remote clients/servers will respond using (NAT IP address, new port #) as destination address

• remember (in NAT translation table) every (source IP address, port #)  to (NAT IP address, new port #) translation pair

• incoming datagrams: replace(NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

NAT ROUTER

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NAT Controversy - Find out why NAT been a topic of debate?