1. NAT/ARP/RARP (Ch 5 & 8)
Dr. Clincy
Lecture

2. NETWORK ADDRESS TRANSLATION (NAT)
Network Address Translation (NAT) allows a site to use a set of private addresses for internal communication and a set of global Internet addresses for communication with another site. The site must have only one single connection to the global Internet through a router that runs NAT software.
The routers only 2 address: (1) the global IP address and (2) one private address
Dr. Clincy, CSIS4500

3. Address translation
All packets leaving the network get assigned the global address as the source address
(straightforward process)
All packets coming into the network get their global destination address replaced with the appropriate private address
(process is more involved)
(explain this in the next ppt slide)
Dr. Clincy, CSIS4500

4. Translation Packet: From Private Network to Internet
Keep in mind that, with in the private network, the original source address is a private address representing the original source in the private network.
Just before the packet leaves the router, the router makes note of the GLOBAL DESTINATION ADDRESS and cross-references it with the PRIVATE source address before changing the private source address to the GLOBAL SOURCE ADDRESS
Packet: From Internet Back to Private Network
When the packet returns, the SOURCE ADDRESS of the packet is the original DESTINATION ADDRESS.
The router uses the new source address of the packet in determining the private destination address – recall the address being cross-referenced
Dr. Clincy, CSIS4500

5. NAT Using Multiple Global Addresses
NAT Router with One GLOBAL address can only allow One private host to access the same EXTERNAL host – with more global addresses, more private hosts can access the SAME external host
A NAT Router with 8 global addresses can allow up to 8 private addresses (hosts) to access the SAME external host (simultaneously) – can create up to 8 separate connections
To create a many-to-many relationship, a 5-column table (versus 2-column table) is needed in reducing uncertainty – by specifying port address and transport layer protocol
Five-column translation table
Dr. Clincy, CSIS4500

6. An ISP and NAT
An ISP serving DIAL-UP customers can conserve addresses by using NAT.
NOTE: think of dial-up customers as being apart of the ISP’s private network before gaining access to the Global Internet.
The ISP could assign a private address to each customer and when the customer leaves the private network, a translation would occur .
Let an ISP with 100,000 dial-up customers be granted only 1000 global addresses - the ISP could assign private addresses to each 100,000 customers and the ISP translate the 100,000 source addresses for the outgoing packets with the 1000 global addresses
Dr. Clincy, CSIS4500

7. ARP & RARP (Ch 8)
Dr. Clincy
Lecture

8. Physical and Logical Addresses
Recall: physical address is needed on a local perspective
Recall: logical (or IP) address is needed on a global perspective
Therefore, both addresses are needed
Likewise, there is a need to map the logical address to it’s corresponding physical address (and vice versa)
The mapping can be “static” or “dynamic” in nature
Dr. Clincy
Lecture

9. RECALL
Going from network A physical address 10 to network P physical address 95.
Can’t use the physical address ONLY because different networks
The network layer address contains the uniqueness we need from source to sink across interconnected networks
Dr. Clincy
Lecture

10. Static Mapping Approach
Static Mapping simply entails creating a table associating the logical address with the physical address
The static-mapping table is stored in each machine on the network
A machine knowing another machine’s IP address can simply look-up the other machine’s physical address
What’s the problem with this ? The physical address could change (ie. (1) NIC could change, (2) some LANS change the physical addresses and (3) laptops (physical address changing physical network to physical network))
By having to update the static tables periodically, performance could suffer
Dr. Clincy
Lecture

11. Position of ARP and RARP in TCP/IP protocol suite
For the dynamic approach: given 1 of the 2 addresses, the other address can be determined
The ARP (Address Resolution Protocol) and RARP (Reverse Address Resolution Protocol) are used to find the other address
Dr. Clincy
Lecture

12. Dynamic Mapping: ARP and RARP
ARP: maps the logical address to the physical address (given logical, find physical)
RARP: maps the physical address to the logical address (given physical, find logical)
ARP/RARP use unicast & broadcast physical addresses (from earlier lectures, recall the formats of the broadcast/unicast addresses)
Dr. Clincy
Lecture

13. ARP operation
Keep in mind that the Tx doesn’t know the physical address of the Rx – and the IP address alone IS NOT enough info to tell the Tx where the RX is located – this is the problem ARP solves
The Tx broadcast out an ARP query packet to all stations on the network – the packet contains the Tx’s physical/IP addresses and the IP address it’s seeking to find a physical address for.
All stations on the network receives the query packet however, only the station with that IP responds
The responding Rx sends back it’s physical address (in a unicast form) to the Tx
Now the Tx can unicast the actual data to the Rx
Why are we particular about when to unicast vs broadcast ?
Dr. Clincy
Lecture

14. ARP packet
Hardware type (HTYPE) – 16 bits – type of network running ARP; each LAN
Protocol type (PTYPE) – 16 bits – defines the protocol (ARP can be used with any higher level protocol) – example: IPv4 vs IPv6
Hardware Length (HLEN) – 8 bits – defines the length of the physical address (bytes)
Protocol Length (PLEN) – 8 bits – defines the length of the logical address in bytes (ie. 4 for IPv4)
Operation (OPER) – 16 bits – defines the type of packet (either ARP request or ARP reply)
(Note: where do you think this info will be in, let’s say an Ethernet Frame ???)
Dr. Clincy
Lecture

15. Encapsulation of ARP packet
The ARP packet is encapsulated into the data link frame we discussed 3 or 4 lectures ago
In this case, the “type” would indicate an ARP packet is being sent in the “data” section of the data link frame
Dr. Clincy
Lecture

16. Four cases in using ARP
The Tx is a host and the Rx is a host with in the same network – therefore, the IP address of the Rx must be mapped to a physical address
The Tx is a host and the Rx is a host on a different network – therefore, the Tx uses the routing table to find the next hop and then the logical address of the router must be mapped to it’s physical address – this is done hop-to-hop
The Tx is a router and the Rx is a host (router) on a different network – therefore, the Tx uses the routing table to find the next hop and then the logical address of the next router must be mapped to it’s physical address – this is done hop-to-hop.
The Tx is a router and the Rx is a host with in the same network – therefore, the IP address of the Rx must be mapped to a physical address
Dr. Clincy
Lecture

17. Example
A host with IP address and physical address 0xB has a packet to send to another host with IP address and physical address 0xA46EF45983AB. The two hosts are on the same Ethernet network. Show the ARP request and reply packets encapsulated in Ethernet frames.
Dr. Clincy
Lecture

18. Solution The Figure below shows the ARP request and reply packets.
Dr. Clincy
Lecture

19. Proxy ARP
Have a router acting on behalf of a set of hosts – Proxy ARP Router
Proxy ARP router figures out the physical address of the host and then it send the address to the appropriate router (or host)
Good for sub-netted cases
This router handles these guys’ ARP requests
These gals’ handle their own ARP requests
Dr. Clincy
Lecture

20. No Need to know about the ARP Package
Skip this section
Dr. Clincy
Lecture

21. RARP
For the ARP case, the IP address of the RX was known and the physical address of the Rx was unknown
The Reverse Address Resolution Protocol is used when the physical address of the Tx is known and the IP address of the Tx is unknown.
Recall: when sending an IP datagram across different networks, the IP address is needed.
Dr. Clincy
Lecture

22. RARP Operation The RARP operation occurs with in the local network
Some Tx needs to know it’s own IP address
That Tx broadcasts a RARP request to all host in the local network – the request contains the Tx’s physical address (the Tx gets the physical address from its NIC)
Some Rx which contains all of the Logical-to-Physical address mappings, unicast back to the Tx the Tx’s IP address
Dr. Clincy
Lecture

23. RARP packet
The format of the RARP packet is the same as the ARP packet however:
Operation will be either a RARP Request or RARP Reply
The sender’s hardware address field is padded until filled
Dr. Clincy
Lecture

24. Encapsulation of RARP packet
Like the ARP Packet, the RARP Packet is embedded into the data link frame
Notice the “type” (8035)
Dr. Clincy
Lecture

25. Dynamic Mapping: ARP and RARP
ARP: maps the logical address to the physical address (given logical, find physical)
RARP: maps the physical address to the logical address (given physical, find logical)
ARP/RARP use unicast & broadcast physical addresses (from earlier lectures, recall the formats of the broadcast/unicast addresses)
Dr. Clincy
Lecture

26. Alternative Solutions to RARP
When a diskless computer is booted, it needs more information in addition to its IP address.
It needs to know its subnet mask, the IP address of a router, and the IP address of a name server.
RARP cannot provide this extra information. New protocols have been developed to provide this information.
In Chapter 18 we discuss two protocols, BOOTP and DHCP, that can be used instead of RARP.
Dr. Clincy
Lecture