IDSC 4490 – Network Security

1. IDSC 4490 – Network Security Networking Review (I) Alok Gupta, Dept. of IDSC

2. Topics • LANs • Bridges • Hubs and Switches • Media Access Control (MAC) address • IP numbering • Domain Names • Network Classes • Subnets • Routing

3. Local Area Networks • LAN topologies • Star • Ring • Bus • Most Used • Star • Ethernet • All computers on a LAN can receive all the transmitted packets

4. Bridges • Why Bridges? • LANs have limitations in terms of the length of ether and the number of computers • Bridges connect different segments of a LAN • Data is sent to the other segment only if the receiving computer is on the other segment

5. Hubs • Look like Star act like Bus • Sometimes called ``Ethernet-in-a-box'' • Effectively a very short Ethernet with very long cables • Can be connected into larger Ethernets

6. Bridge • With switching, multiple stations can transmit simultaneously • Provides much higher aggregate bandwidth Switches • Effectively a separate LAN segment for each port • Similar to hub - hub shares single segment among all ports

7. NICs and MACs • Computer systems use special purpose hardware for network connection • Typically a separate card in the back panel • Network adapter card or network interface card (NIC) • Each card usually has a unique hardware number often referred to as Media Access Control (MAC) address • Computers on a LAN send the message to each other using MAC addresses • Each MAC address is unique

8. Internet -- Glossary • An internet is a collection of physical networks interconnected into a single virtual network • Routers provide the physical interconnection and forward packets between networks • Hosts communicate across multiple network through packets forwarded by routers • TCP/IP is the most widely used internetworking protocol suite

9. IP Address • IP numbers are the addressing scheme in TCP/IP protocol suite • Each host is assigned a 32-bit number • This number is called the IP address or Internet address • This number is unique across the entire Internet

10. IP Address -- Characteristics • Each IP address is divided into a prefix and a suffix • Prefix identifies network to which computer is attached • Suffix identifies computer within that network • Address format makes routing efficient

11. Domain Names • For ease of human reference, Domain Names can be assigned to each network/computer • Domain names are mapped to IP numbers for actual routing • The translation of domain names to IP number is done by Domain Name Service (DNS) • Domains can be defined in a hierarchical manner, for example • .edu • umn.edu • csom.umn.edu • Ids.csom.umn.edu

12. IP Address -- Examples • Domain umn.edu • 160.94.0.0 – 160.94.255.255 • 160.94.254.1  www1.umn.edu • 131.212.0.0 – 131.212.255.255 • 131.212.109.39  www.d.umn.edu • 146.57.0.0 – 146.57.255.255 • 146.57.6.12  www.mrs.umn.edu • Domain csom.umn.edu • 160.94.112.0 – 160.94.119.255 • 160.94.113.243  ids.csom.umn.edu *In red are network prefix – the rest are either subnet prefixes or IP numbers assigned to computers

13. IP Address – Binary Representation 128 64 32 16 8 4 1 128 64 32 16 8 4 1 128 64 32 16 8 4 1 2 2 2 128 64 32 16 8 4 1 2 1 1 0 0 1 0 0 1 0 1 0 1 0 0 0 1 1 1 0 1 1 1 1 0 0 0 0 1 1 1 0 1

14. Global Design of IP Numbers • There are a total of 32-bits • We can allocate some bits for prefix (network number) and some for suffix (host numbers) • Large prefix, small suffix—many networks, few hosts per network • Small prefix, large suffix—few networks, many hosts per network • Because of variety of usages, organizational sizes, we need to allow for both large and small networks

15. Classes of IP Numbers • Class of an address is identified by first four bits

16. Usage of IP Classes • Class A, B and C are primary classes • Used for ordinary host addressing • Class D is used for multicast, a limited form of broadcast • Internet hosts join a multicast group • Packets are delivered to all members of group • Routers manage delivery of single packet from source to all members of multicast group • Used for mbone (multicast backbone) • Class E is reserved

17. Recognizing a Network’s Class • While dotted decimal makes separating network address from host address easier, determining class is not so obvious • Look at first dotted decimal number, and use this table:

18. Number of Networks & Hosts • Example for Class A: • First bit must be 0 • 7 remaining bits identify Class A net • 27 (= 128) possible class A nets

19. Dividing A Network • It is not always efficient to use the whole IP block assigned to an organization from a central place. For example • UMN has many different units that need to manage their own networks, e.g., CSOM • This is done by assigning a block of IP numbers to CSOM • The way to identify this block is by a network number and a subnet mask • A subnet mask is also a 32-bit number with a continuous series of 1’s up to a point, for example Or 255.255.248.0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0

20. Identifying a Network • Every network in a TCP/IP internet is assigned a unique network number • Usually yyy.yyy.yyy.0 or yyy.yyy.yyy.1 • Each host on a specific network is assigned a host number or host address that is unique within that network • Host's IP address is the combination of the network number (prefix) and host address (suffix)

21. Identifying a Network Number from a Host Number • By performing an AND operation at individual bit level using a Host’s IP Number and the Subnet Mask associated with that IP number. • For example www.csom.umn.edu has an IP number of 160.94.119.32 and the Subnet Mask associated with it is 255.255.248.0 Bit Value 128 64 32 16 8 4 1 128 64 32 16 8 4 1 128 64 32 16 8 4 1 2 2 2 128 64 32 16 8 4 1 2 IP Number 1 0 1 0 0 0 0 0 1 0 1 1 1 0 0 1 1 1 0 1 1 0 1 1 0 0 1 0 0 0 0 0 Subnet Mask 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 1 1 0 0 0 0 0 0 0 0 0 Network Number 1 0 1 0 0 0 0 0 1 0 1 1 1 0 0 1 1 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 In Decimal Notation: 160.94.112.0

22. IP Address and MAC Address • A computer’s IP number can change over time, however, its MAC address remains the same. • To actually send (route) packets to a given computer, IP numbers have to be mapped to a particular MAC address.

23. Router • A router is a computer with multiple IP addresses so that it can be part of multiple networks • If a router is connected to different types of networks (ethernet and token ring, for example) it must have multiple NICs

24. Router -- Example

25. Routing -- Concept • For efficiency, information about forwarding is stored in a routing table • Initialized at system initialization • Must be updated as network topology changes • Contains list of destination networks and next hop for each destination

26. Routing – Practice • In practice, additional information is kept in routing table • Destination stored as network address • Next hop stored as IP address of router • Address mask defines how many bits of address are in prefix • Prefix defines how much of address used to identify network • E.g., class A mask is 255.0.0.0 • Used for subnetting

27. Routing -- Example A packet with destination 192.4.10.76 and a subnet mask of 255.255.255.0 arrives => it’s destination is 192.4.10.0

28. Next Class • How do we know whether its an email packet, Http packet, telnet packet, …? • What happens if a router goes down or doesn’t receive the send packets? • How do we know when to change routing tables? • Can we detect busy spots on the network? • How does a router find a computer when that computer moves its location?