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

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

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

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

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

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

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

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

9 Network Layer Header

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

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

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

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

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

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

16 IP Class A n Format: Network.Host.Host.Host. n First octet begins with 0 (most significant bit). n First octet has decimal values from 0 to 126. u Note 127 is reserved for special purpose. n Class A is a very large network. u 2 7 class A networks (128, only 126 are valid) u 2 24 = 16,777,216 hosts per network (16,777,214 valid - all 0s and all 1s are not assigned).

17 IP Class B n Format: Network.Network.Host.Host n First octet begins with 10 (most significant bits). n First octet has decimal values from 128 to 191. n Class B is an intermediate size network. u 2 14 class B networks. u 2 16 = 65,536 hosts per network (65,534 valid).

18 IP Class C n Format: Network.Network.Network.Host n First octet begins with 110 (most significant bits). n First octet has decimal values from 192 to 223. n Class C is a small network. u 2 21 networks. u 2 8 = 256 hosts per network (254 valid).

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

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

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

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

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

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

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

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

27 Router’s use of Subnet Mask n 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.

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

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

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

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