07/25/18

Routers need at least 2 IP addresses and 2 MAC addresses
- every router needs to have at least the number of networks it is part of
Introducing NAT and PAT
- special types of routers
- NAT = network address translation
- PAT = port address translation
- NAT = circle with 2 arrows coming out and 2 arrows coming in
- has a real IP address and one that is private
- replaces private with real IP when talking to internet
- router lives on multiple levels of internet
- allows security to be downloaded only on one router
- private IP addresses = 192.168.?.? , 10.?.?.? , 172.?.?
DHCP: Dynamic Host Configuration Protocol
- automatic assignment of an OP address to an indicidual host
- extension and generalization of old BOOTP program
- normally layer-3 stuff is subject to error, but DHCP has aspects that
  - recover from loss or duplication
  - cache server addresses
  - avoid synchronization flooding
- has a message format
ROUTING (especially non NAT PAT routers)
- router delivers directly to the ones they are a member of
- otherwise, delivers to another router
- check each destination from smallest mask to largers
- routers change headers, never change IP addresses in datagram
- change MAC addresses
- encapsulation - add header
- decapsulation -
ARP - Address Resolution Protocol
- how IP addresses to MAC address
- a host or router uses ARP to resole an OP address to a MAC address
- a computer never attempts to resolve the address of a computer that attaches to a remote network
- cache- copy of things used recently, small local scratch space
- ARP is considered data in a 802 or ethernet packet
- forms boundary in the TCP / IP stack
- MAC addresses used in lower 2 levels
- IP addresses used in upper 3 levels
IP header
- Type: error, tcp, udp
- service type: voiceover IP, streaming video, "how serious is it if we lose this data"
Sometimes datagrams are broken into pieces
MTU: Maximum transmission unit
- flags + fragment offset
Vers
- which version of IP address
IP header + Data = ethernet data
- all 1's and 0's
- MAC addresses are in frame header

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