Outline Lecture 10. Subnetting Variable Length Subnet Mask (VLSM) Subnetting & Supernetting Supernetting Classless Inter-Domain Routing (CIDR) Giuseppe Bianchi Giuseppe Bianchi medium org: N x class C? Class B? Need for subnetting 130.11.0.7 213.2.96.0 Class C addresses: Net Net_id-Host_id: Undersized (254 R2 130.11.0.0 R3 hosts) 213.2.97.0 place host_id on physical network net_id Class B addresses: M uch m ore than R2 Routing Table 131.175.0.2 131.175.0.3 131.175.45.54 131.175.255.254 213.2.98.0 enough (65534 hosts) dest Next Hop N x class C: 130.11.0.0/16 Direct fwd Unw ise: exponential … … 213.2.99.0 grow th of routing 213.2.96.0/24 130.11.0.7 tables 131.175.0.1 213.2.97.0/24 130.11.0.7 Corporate R esult: Class B 65534 hosts on a same physical network???? 213.2.98.0/24 130.11.0.7 CLASS B: addresses w ere - performance? 213.2.99.0/24 130.11.0.7 From: 131.175.0.1 - management? largely preferred To: 131.175.255.254 T he afterm ath: 10 bit class C design w ould have been m uch better… Giuseppe Bianchi Giuseppe Bianchi Idea: further hierarchy level Subnet creation and management subdivide a network in several subnetworks Give me a class B, please Private each subnet = a physical network (Ethernet, FDDI, X.25, 131.175.0.0 for you! Network ATM, Frame Relay, etc….) InterNIC Administrator 131.175.21.42 131.175.12.33 131.175.12.34 131.175.12.0 131.175.21.4 131.175.12.12 131.175.12.0 131.175.12.254 113311.1.17755.2.211.0.0 Internet 131.175.15.0 Router Ethernet FDDI 131.175.15.0 131.175.33.0 113311.1.17755.x.x.0.0 Host 131.175.0.0 Best for local adm inistrator: 131.175.12.0 flexibility to create new netw orks w ithout asking InterN IC 131.175.21.0 131.175.21.1 new classfuladdresses. Sub-Net ATM Best for Internet: Class B network: 131.175.0.0 R oute flapping in the private dom ain do not affect Internet May use third byte to identify subnet: 131.175.X.0 (or may not!) O ne single entry in core router tables address all subnetw orks Giuseppe Bianchi Giuseppe Bianchi 1 Subnetting Subnet Address & Mask Class B address example H ost IP address: 159.100.9.18 10011111.01100100.00001001.00010010 network prefix Class B - netw ork m ask: (network address) 255.255.0.0 11111111.11111111.00000000.00000000 S ubnet M ask 1 0 NET ID (14bit) HOST ID (16 bit) Longer than natural class m ask; Length set by adm inistrator T ells w here the boundary netw ork-host really is Exam ple: class B address w ith 5 bits subnet_id Extended network prefix subnet m ask = /21 11111111.11111111.11111000.00000000 (subnet address) /prefix-length notation subnet m ask = 255.255.248.0 (dot decimal notation) 1 0 NET ID (14bit) SUBNET ID (n bit) HOST ID (16-n bit) 159.100.0.0 = net_id 10011111.01100100.00001000.00000000 159.100.8.0 = extended netw ork address (net_id+subnet_id) T o avoid am biguity: 159.100.8.0/21 Giuseppe Bianchi Giuseppe Bianchi Remember: subnetting is arbitrary! Typical class B subnetting Example: subnetting Class C 193.1.1.0 Address Class B address = /16 network prefix Base net 11000001.00000001.00000001.00000000 193.1.1.0/24 Class C network address = 131.175.0.0 1 1 0 NET ID (21bit) HOST ID (8 bit) /24 prefix natural mask = 255.255.0.0 Subnetted Subnet Host id 255.255.255.224 1 1 0 NET ID (21bit) Subnetted with /24 network prefix (3 bit) (5bit) /27prefix 1 0 NET ID (14bit) SUBNET ID (8 bit) HOST ID (8 bit) Subnet # 0 11000001.00000001.00000001.00000000 193.1.1.0/27 Subnet # 1 11000001.00000001.00000001.00100000 193.1.1.32/27 255.255.255.0 subnet mask Subnet # 2 11000001.00000001.00000001.01000000 193.1.1.64/27 Subnet # 3 11000001.00000001.00000001.01100000 193.1.1.96/27 subnet ID = third number in dotted notation Subnet # 4 11000001.00000001.00000001.10000000 193.1.1.128/27 131.175. .0 Subnet # 5 11000001.00000001.00000001.10100000 193.1.1.160/27 21 Subnet # 6 11000001.00000001.00000001.11000000 193.1.1.192/27 No technical reasons to use /24 subnets, but convenient for humans Subnet # 7 11000001.00000001.00000001.11100000 193.1.1.224/27 (subnet boundary clearly visible in dotted notation) Remember: maximum 30(25-2) hosts attachable to each subnet Giuseppe Bianchi Giuseppe Bianchi Example: route 193.205.102.36 193 205 102 36 Possible netmask values 1 1 0 0 0 0 0 1 1 1 0 0 1 1 0 1 0 1 1 0 0 1 1 0 0 0 1 0 0 1 0 0 Class C address; 128 64 32 16 8 4 2 1 Outside private domain routed with mask 255.255.255.0 1 0 0 0 0 0 0 0 = 128 network host 1 1 0 0 0 0 0 0 = 192 193 205 102 36 1 1 0 0 0 0 0 1 1 1 0 0 1 1 0 1 0 1 1 0 0 1 1 0 0 0 1 0 0 1 0 0 1 1 1 0 0 0 0 0 = 224 1 1 1 1 0 0 0 0 = 240 Inside private domain, administrator has set netmask 255.255.255.248 255 255 255 248 1 1 1 1 1 0 0 0 = 248 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 1 1 1 1 1 1 0 0 = 252 Hence, route to subnet address and then to host id, computed as: 1 1 1 1 1 1 1 0 = 254 network subnet host 1 1 1 1 1 1 1 1 = 255 1 1 0 0 0 0 0 1 1 1 0 0 1 1 0 1 0 1 1 0 0 1 1 0 0 0 1 0 0 1 0 0 193.205.102.32 /29 4 Giuseppe Bianchi Giuseppe Bianchi 2 Subnet routing – 2nd example Router configuration Core routers unaware of subnetting – route via class mask Classful routing: Net = 162.12.0.0; subnet mask 255.255.255.224 … … … … … … 162.12.0.0 193.1.1.36 162.12.0.0 145.54.3.5 162.12.0.0 145.54.3.5 All necessary information To other nets … … … … … … included in Ipaddr 162.12.2.33 Subnet routing 162.12.34.75 145.54.55.1 193.1.1.36 Specific subnet mask 162.12.1.11 To other … … 162.12.1.0 subnets (set by admin) required 162.12.1.12 114455.5.544.0.0.0.0 145.54.3.5 162.12.34.64 162.12.1.1 162.12.1.1 119933.1.1.1.1.0.0 162.12.2.32 162.12.1.33 Routing Table default 162.12.9.65 Subnet mask: 162.12.34.65 255.255.255.224 162.12.70.96 193.1.1.1 dest Next Hop 162.12.70.96 162.12.1.1 162.12.1.33 162.12.9.65 162.12.1.0 Direct fwd 162.12.2.33 162.12.34.66 116622.1.122.3.355.1.12288 … … 162.12.34.64 Direct fwd 162.12.34.75 162.12.34.64 162.12.0.0 193.1.1.36 162.12.35.128 162.12.34.66 … … Net = 162.12.0.0 162.12.70.96 162.12.1.12 routing tables in the 131.175.0.0 162.12.34.66 Internet: subnet mask = 255.255.255.224 May be quite a complex To 131.176.0.0 162.12.34.66 route according to net_id Corporate routers & hosts: Routing table… 131.175.0.0 default 162.12.1.11 Use natural class mask Route according to subnet_id VLSM will help (later) 131.176.0.0 Need to KNOW subnet mask Giuseppe Bianchi Giuseppe Bianchi Subnetting Example (problem) Subnetting Example (solution?) algebra algebra 12 hosts 193.1.1.32/27 up to 30 hosts Computation A A Computation (33-62) 193.1.1.64/27 Math dept 28 host Math dept up to 30 hosts 22 hosts 193.1.1.96/27 (65-94) Link-1 up to 30 hosts Link-1 B (97-126) B physics physics 193.1.1.160/27 10 host up to 30 hosts (161-190) Link-2 Link-2 C C Where are the errors? 193.1.1.0 network 193.1.1.0 network Giuseppe Bianchi Giuseppe Bianchi Subnetting Example (solution!) algebra 193.1.1.32/27 up to 29 hosts Computation (33-62) A 193.1.1.64/27 Math dept up to 29 hosts VLSM 193.1.1.96/27 Link-1 (65-94) up to 29 hosts 193.1.1.128/27 (97-126) B physics Variable Length Subnet Mask 193.1.1.160/27 Subnet mask: up to 28 hosts RFC 1009 (1987) /27 255.255.255.224 (161-190) SUBNETS: Math 193.1.1.96/27 Link-2 193.1.1.192/27 Algebra 193.1.1.32/27 Physics 193.1.1.160/27 Comput 193.1.1.64/27 C Link-1 193.1.1.128/27 Link-2 193.1.1.192/27 193.1.1.0 network --- 193.1.1.0/27 --- 193.1.1.224/27 Giuseppe Bianchi Giuseppe Bianchi 3 A typical problem Variable Length Subnet Mask allows more than one subnet mask in the A same network pc-net x-net-1 100 host 20 host A) more efficient use of organization’s IP address space Link-1 Subnets may significantly vary in relative size (computer B room = 200 hosts, secretary = 4 hosts…) Link-3 consider a 4 host network with mask 255.255.255.0: wastes 250 IP addresses! Link-2 B) allows route aggregation, thus reducing routing ws-net x-net-2 20 host 10 host information needed C Needs further support by routing protocol e.g.