Layer 3: Routing & Addressing

1. Layer 3: Routing & Addressing Honolulu Community College Cisco Academy Training Center Semester 1 Version 2.1.1

2. Overview • Network layer is responsible for routing, navigating the data through the network. • best path selection, and switching the data to the destination. • Router use and operation. • IP addressing. • Classes A, B & C for general use. • Classes D & E are reserved. • Subnetting • subnetworks and subnet masks. • IP addressing schemes.

3. Network Layer Addressing • Network layer's addressing scheme is used to: • represent the path. • determine the destination of data. • Host addresses on internetwork must be unique. • Network addressing is hierarchical, has logical structure. Addresses represent path of media connections.

4. Communications among Networks • The Internet is a collection of interconnected networks or autonomous systems. • Autonomous system - network under a common administration, using a common routing strategy. • Internet service providers (ISPs) offering services • that tie together multiple networks.

5. Routers • Routers are OSI Layer 3 (the network layer) that interconnect, networks or subnetworks. • They pass data packets between networks based on network (layer 3) protocol information. • Routers selects best path for the delivery of data and then direct packets to the appropriate output port and segment. • routing is often referred to as Layer 3 switching. • Recall: routers do (1)path selection; (2)communicate path information; (3) switching function.

6. Path Determination • One of the primary functions of network layer is path determination. • process to choose the next hop in the path. • also called routing the packet.

7. Network Layer Addressing • In general, of two parts: • network address. • host (node) address. • Network address used by router to identify a path within network cloud and/or destination network. • Host or node address identifies the specific port or device.

8. Flat vs Hierarchical Addressing • Network address • Hierarchical, has logical structure. (IP) • like zip code or area code. • Identifies the network. • Host address • Flat addressing scheme (like MAC address).

9. Network Layer Header

10. Network Layer Header (2) • VERS - IP version, currently 4. • HLEN - header length. • Service Type - specifies the level of importance, assigned by a particular upper-layer protocol. • Total Length - length of entire IP packet (bytes). • Identification, Flags, Frag Offset - provides for fragmentation of packets ( for differing MTUs). • Time to Live (TTL) - time packet is considered valid, after this packet is discarded. • Protocol - upper layer (4) protocol.

11. Network Layer Header (3) • Header checksum - CRC or FCS, for error checking, helps ensure IP header integrity. • Source & Destination IP addresses. • IP Options - network testing, debugging, security, etc. • Data - segment from transport layer. • Padding - extra zeros are added to ensure that the IP header is always a multiple of 32 bits.

12. What is IP Addressing? • IP address is a logical address, configured in software. • Each device has a unique IP address. • IP address is a 32 bit number. • Represented as 4 groups of 8 bits (octets or bytes). • Usually written as dotted decimal.

13. Dotted Decimal Format • Consider an example IP address (32bits): • 11010011.00101101.01110011.00010011 • We need to find a decimal value for each octet. • 11010011(binary) equals 211 (decimal). • 00101101(binary) equals 45 (decimal). • 01110011(binary) equals 115 (decimal). • 00010011(binary) equals 19 (decimal). • In dotted decimal, the IP address is: • 211.45.115.19 • What is the largest decimal value for any octet?

14. IP Addressing • IP address is structured or hierarchical. • Network ID - host ID (2 level). • Network ID identifies the network. • Host ID identifies the actual device or node. • Divided into 5 classes. • Class A - net.host.host.host • Class B - net.net.host.host • Class C - net.net.net.host • Class D & E - used for multi-casts and experimental purposes.

15. IP Addressing (2) • Organizations can obtain IP addresses from ARIN (American Registry for Internet Numbers). • Classes A, B, and C are available. • Class A addresses - for governments around the world. • Class B addresses - for medium-sized companies. • Class C addresses - for all others.

16. IP Class A • Format: Network.Host.Host.Host. • First octet begins with 0 (most significant bit). • First octet has decimal values from 0 to 126. • Note 127 is reserved for special purpose. • Class A is a very large network. • 27 class A networks (128, only 126 are valid) • 224 = 16,777,216 hosts per network (16,777,214 valid - all 0s and all 1s are not assigned).

17. IP Class B • Format: Network.Network.Host.Host • First octet begins with 10 (most significant bits). • First octet has decimal values from 128 to 191. • Class B is an intermediate size network. • 214 class B networks. • 216 = 65,536 hosts per network (65,534 valid).

18. IP Class C • Format: Network.Network.Network.Host • First octet begins with 110 (most significant bits). • First octet has decimal values from 192 to 223. • Class C is a small network. • 221 networks. • 28 = 256 hosts per network (254 valid).

19. IP Reserved Addresses • For each network, two addresses are reserved. • All 0s host ID - used as part of the network address or wire address. • All 1s host ID - used as part of the broadcast address. • For class B network address 150.20.0.0, note all host bits are 0s. • Directed broadcast address is 150.20.255.255, all host bits are 1s. • Local broadcast address is 255.255.255.255, all bits are 1s.

20. Significance of Network ID • Hosts on a network can only communicate directly with devices with the same network ID. • Even if they are physically connected, they must have same network ID to communicate directly. • Note: the rest of the world sees our network as a single network, represented by the network address.

21. IP Address Range • Class C 200.10.10.0 • 24 bits (first 3 octets) are network ID, 8 bits (4th octet) are host bits. • For all possible host combinations, we get 200.10.10.0 to 200.10.10.255. • First address is the network address or wire address (host ID all 0s). • Last address is the network broadcast address (host ID all 1s). • Allowable host addresses are 200.10.10.1 to 200.10.10.254.

22. IP Address Range (2) • Remember: • 1) First address in each network is reserved for the actual network address (or network number). • 2) The last address in each network is reserved for broadcasts. • 3) The rest of the addresses are assignable to various hosts.

23. Subnetting • To provide addressing flexibility, networks, particularly large networks,need to be divided into smaller networks, called subnetworks or subnets. • Subnet addresses are assigned locally, usually by the network administrator. • Each subnet address must be unique.

24. Subnetworks • To create a subnet address, bits are borrowed from the original host portion and designated as the subnet field. • Primary reason for using subnets is to reduce the size of a broadcast domain.

25. Subnetworks (2) • Minimum no. of bits for subnet is 2. • All 0s subnet, and all 1s subnet are no assigned. • 2n-1 usable subnets, where n = no. of subnet bits. • Note: Number of host bits are less. • Fewer users per subnet (segmentation). • Network bits (ID) are unchanged. • Rest of the world still sees our network as a single network (network address).

26. Subnet Mask • Subnet mask (formal term: extended network prefix), tells network devices which part is network field and which part is host field. • Subnet mask is 32 bit (dotted decimal. • All network and subnet bits are set to 1s. • All host bits are set to 0s.

27. Router’s use of Subnet Mask • To route a data packet, the router must first determine destination network/subnet address by performing a logical AND using destination host's IP address and subnet mask. Class B, 8 bits borrowed.

29. Subnet Bits • Minimum number of subnet bits is 2. • Maximum varies by address class. • Address Size of Default Maximum Number • Class Host Field of Subnet Bits • A 24 22 • B 16 14 • C 8 6 • Must have min of 2 host bits. • Some devices can use the zero subnet.

30. Private Addresses • Certain addresses in each class of IP address that are not assigned - private addresses. • used with network address translation (NAT), or with a proxy server, to connect to a public network. • can be used in stand-alone network. • Private address ranges will NOT be routed on the Internet.

31. Summary • Network layer functions include network addressing and best path selection. • Two addressing methods: • MAC addressing which is flat scheme. • IP or network addressing is hierarchical. • Class A, B, and C are used for Internet addressing. • Cass A addresses - for governments. • High-order bit is 0; first octet - 1 to 126. • Class B addresses - for medium-size companies. • 10 is high-order bit; first octet - 128 to 191.

32. Summary (2) • Class C addresses - for all others. • 110 is high-order bit; first octet - 192 to 223. • Subnetting - adding additional logical structure to network addressing. • Bits are borrowed from host to create subnet. • Subnet mask - indicates which bits are network and subnet, and which are hosts. • Private IP Addresses • Class A: 10.0.0.0, subnet mask 255.0.0.0 • Class B: 172.16.0.0, subnet mask 255.240.0.0 • Class C: 192.168.0.0, subnet mask 255.255.0.0 The End