VLANs and DHCP

Dr. Alexios Louridas

Virtual LANs

  • One user moves office, but would need to connect to a different switch?
  • single broadcast domain:
    • all layer-2 broadcast traffic must cross entire LAN
    • security/privacy, efficiency issues

Port-based vlan

switch ports grouped (by switch management software) so
single physical switch ……

  • 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

How to get an IP Address

  • Hard Coded
    • Configuration Files
  • Auto Allocate
    • Dynamic Host Configuration Protocol (DHCP)

  • 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.

DHCP not only IP

  • Subnet Mask
    • Network vs Host 
  • Default Gateway
  • DNS server (Name and IP address)
  • IP address for first hop router

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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?

1

Privacy and Security Concerns:

  • IP address hiding
  • Security Implications

2

Breaking End to End connectivity

3

Scarcity of IPv4 Addresses

Perpetuates the use of IPv4

5

Impact on Peer-to-Peer Applications

4

Complexity and Maintenance

 

6

Use of Large Scale NATS (LSN) or sometimes called CarrierGrade NAT (CGN)