CIT 384 Network Administration Subnetting CIT 384 Network

CIT 384: Network Administration Subnetting CIT 384: Network Administration 1

Topics 1. 2. 3. 4. 5. IP Addresses Classful and classless addressing Subnet Masks and Prefixes Subnet Math Subnet Problems CIT 384: Network Administration 2

IP Addresses 32 -bit integers One for each network interface. Dotted decimal notation: ii. jj. kk. ll 172 10101100 . 16 00010000 . 254 11111110 . 1 00000001 1 byte 32 bits = 4 bytes CIT 384: Network Administration 3

Grouping IP Addresses Groups of consecutive IP addrs are called networks. Routing table would only need 3 entries below. CIT 384: Network Administration 4

Network and Host Parts IP addresses are divided into two parts – Network ID (like zip code) – Host ID (like street address) Network ID Host ID Two special IP addresses – Network address (e. g. 130. 4. 0. 0) – Broadcast address (e. g. 130. 4. 255) CIT 384: Network Administration 5

Address Classes Class A: 0. 0 -127. 255 8 -bit net ID, 24 -bit host ID 224 – 2 hosts per network; 126 networks Class B: 128. 0. 0. 0 -191. 255 16 -bit net ID, 16 -bit host ID 216 – 2 hosts per network; 16, 384 networks Class C: 192. 0. 0. 0 -223. 255 24 -bit net ID, 8 -bit host ID (28 – 2) = 254 hosts per network; 2, 097, 152 networks Class D: 224. 0. 0. 0 -239. 255 28 -bit multicast group ID Class E: 240. 0 -255. 255 Reserved for future use CIT 384: Network Administration 6

CIDR Classless Inter-Domain Routing – Classful routing wastes most IP addresses. – Allocate addresses on bit boundaries instead of byte boundaries. – Allow ISPs/users to decide on boundaries instead of basing on IP addresses. Prefix notation – /x indicates that first x bits are shared. – 192. 168. 0. 0/16 = 192. 168. 0. 0 – 192. 168. 255 CIT 384: Network Administration 7

Public IP Addresses ICANN assigns network numbers. – Internet Corporation for Assigned Network Numbers. – ICANN gives authority to regional orgs, e. g. ARIN (American Registry for Internet Numbers) – Typically to ISPs, universities, corporations. ISP assigns IP addresses within network CIT 384: Network Administration 8

Private RFC 1918 IP Addresses Private IP Networks 10. 0 172. 16. 0. 0 through 172. 31. 0. 0 192. 168. 0. 0 through 192. 168. 255. 0 Network Class Count of Networks A 1 B 16 C CIT 384: Network Administration 256 9

IPv 4 vs IPv 6 Addresses Feature Size of Address Example Address IPv 4 32 bits 10. 1. 1. 1 IPv 6 128 bits 0000: FFFF: FFFF : 0 A 01: 0101 Abbreviated Address Localhost Possible Addresses - : : FFFF: 0 A 01: 0101 : : 1/128 2128 (~3. 4 x 1038) 127. 0. 0. 1 232 (~4 billion) CIT 384: Network Administration 10

Network Mask How do we list subnets in routing table? – Ex: addresses 150. 4. 0 – 150. 4. 255 – Table: 155. 4. 0 netmask 255. 0 Subnet mask indicates range – Binary 1 s indicate network part of address. – Binary 0 s indicate host part of address. – Always consists of 1 s followed by 0 s. Prefix notation – Humanly readable form of subnet mask. – Just counts the number of binary 1 s in mask. CIT 384: Network Administration 11

Classful Address Ranges and Masks Class A NNNN. HHHHHHHH Class B NNNNNNNN. HHHH Class C NNNNNNNN. HHHH Class Leading Bits Start End Subnet Mask CIDR A 0 0. 0 126. 255. 0. 0. 0 /8 B 10 128. 0. 0. 0 191. 255. 0. 0 /16 C 110 192. 0. 0. 0 231. 255. 0 /24 D 1110 224. 0. 0. 0 239. 255 N/A E 1111 240. 0 255. 0 N/A CIT 384: Network Administration 12

Example • • IP Address: 137. 201. 18. 42 Address Class: B since 128 < 137 < 191 Default Netmask: 255. 0. 0 Network Address Part: 137. 201. 0. 0 Host Address Part: 0. 0. 18. 42 Broadcast Address: 137. 201. 255 Host Address Range for Network: – 137. 201. 18. 1 through 137. 201. 255. 254 CIT 384: Network Administration 13

Why Subnet? Allows admin to create more networks for: 1. 2. 3. 4. 5. Address conservation. Organization of hosts. Different physical media. Security. Performance (smaller broadcast domains. ) CIT 384: Network Administration 14

IP Addresses with Subnets Route on network + subnet part of address. CIT 384: Network Administration 15

Subnet Math Binary <-> Decimal Conversion – Convert each byte of dotted quad into binary. – Convert binary byte into 4 decimal values. Boolean AND operation – – 0 AND 0 = 0 0 AND 1 = 0 1 AND 0 = 0 1 AND 1 = 1 Convert between dotted quad and prefix. – 255. 0 netmask is identical to /24 CIT 384: Network Administration 16

How to find network address? (IP Address) AND (Subnet Mask) 137. 201. 18. 42 AND 255. 0. 0 10001001. 11001001. 000100101010 11111111. 0000 -------------------------10001001. 11001001. 00000000 (convert from binary to decimal) 137. 201. 0. 0 CIT 384: Network Administration 17

How to find number of networks? Address divided between network and host. – If there are s subnet bits and h host bits, then – Number of subnets = 2 s – Number of hosts = 2 h – 2 Subnet zero – Classful routing reserves 2 subnets so only have 2 s – 2. • Lowest and highest subnet numbers. – For Class B network 150. 0. 0 reserves • 150. 0. 0 (ambiguity with address of whole B) • 150. 255. 0/24 (ambiguous broadcast 150. 255) CIT 384: Network Administration 18

Choosing Subnet Mask to meet Design Requirements – Class B network 130. 1. 0. 0 – Number of subnets: 200 – Max hosts per subnet: 200 Problem 1: how many host bits? – Find h, # of host bits, such that 2 h >= 200. – 27 = 128, 28 = 256, therefore h = 8. Problem 2: how many subnet bits? – Find s, # of subnet bits, such that 2 s >= 200, yields s = 8. – NNNNNNNN. SSSS. HHHH Problem 3: find subnet mask – 11111111 0000 – 255. 0 CIT 384: Network Administration 19

Multiple Possible Subnet Masks In some problems, many subnet masks exist. Ex: change # of subnets from 200 to 50. – Find s, # of subnet bits, such that 2 s >= 50, yields s = 6. – NNNNNNNN. SSSSSSxx. HHHH – How many possible subnets exist? • • 1111111100 (6 subnet bits, 10 host bits) 111111110 (7 subnet bits, 9 host bits) 11111111 (8 subnet bits, 8 host bits) 1111111101 (impossible) – Subnet masks • /22 255. 252. 0 (6 subnet bits, 10 host bits, 1022 hosts/sub) • /23 255. 254. 0 (7 subnet bits, 9 host bits, 510 hosts/sub) • /24 255. 0 (8 subnet bits, 8 host bits, 254 hosts/sub) Do you want to maximize # subnets or # hosts/subnet? CIT 384: Network Administration 20

References 1. James Boney, Cisco IOS in a Nutshell, 2 nd edition, O’Reilly, 2005. 2. Cisco, Cisco Connection Documentation, http: //www. cisco. com/univercd/home. htm 3. Cisco, Internetworking Basics, http: //www. cisco. com/univercd/cc/td/doc/cisintw k/ito_doc/introint. htm 4. Matthew Gast, 802. 11 Wireless Networks: The Definitive Guide, O’Reilly, 2005. 5. Wendell Odom, CCNA Official Exam Certification Library, 3 rd edition, Cisco Press, 2007. CIT 384: Network Administration 21