HW2 Answers NET 413 Fall 2023

NET 413 HW 2 Answers Due Sunday, Oct. 8, 2023 by 11:59 pm 50 points Name: Dale Buchholz Submit your answers to the following questions on D2L. Include your name at the beginning of your answer file. 1. (2 pts) My IP address is 140.62.52.18 and my subnet mask is 255.255.255.248. What are the first and last assignable IP addresses in my subnet? Recall that subnet address has all zeroes for host ID and broadcast has all ones as host ID. The assignable addresses are in between the subnet and broadcast addresses. (see slides 23-27 in 2IP Subnetting) 255.255.255.248 is equivalent to /29. Subnet size is 8. There are (32-29) = 3 Host bits in each address. First, determine Network Address by (a) zeroing out last 3 bits of “18” which gives “16” or (b) determine greatest multiple of 8 that is less than 18, which is 16. In either case, the Network address is 140.62.52.16 and the Subnet ID is 140.62.52.16/29. The First and Last Assignable IP addresses of 140.62.52.16/29 are 140.62.52.17 and 140.62.52.22 2. (2 pts) My IP address is 140.62.52.18 and my subnet mask is 255.255.248.0. What are the first and last assignable IP addresses in my subnet? 255.255.248 .0 is equivalent to /21. Subnet size is 2048 since there are (32-21) = 11 Host bits in each address. First, determine Network Address by (a) zeroing out all of 4 th byte and the last 3 bits of 3 rd byte “52” which gives “48” or (b) 3 rd byte Jump Factor is 8 (equals 2048/256 or 256-248), so for 3 rd byte we determine greatest multiple of 8 that is less than 52, which is 48. In either case, the Network address is 140.62.48.0 and the Subnet ID is 140.62.48.0/21. The First and Last Assignable IP addresses of 140.62.48.0/21 are 140.62.48.1 and 140.62.55.254 3. Company A has been allocated the Class B address block 177.248.0.0/16. Internally, they want to break this address block into 32 subnets of equal size. a) (2 pts) What Subnet Mask should be used on internal devices to achieve this? What power of 2 is greater than or equal to 32? That is 2 n = 32? What is the value of n? Using slide 21, the Network Prefix is the first 16 bits (188.253), the subnet ID uses the next n bits, and the host ID uses the remaining bits (32-16-n). Here are two ways to do this problem: i) Since they need 32 subnets and 2 5 = 32, they need to borrow 5 bits, so the new prefix length is 16+5 = 21. The new subnets will be /21, which is 255.255.248.0 . ii) They have 65,536 addresses in their /16 block. To split into 32 subnets, each will have size 65,536/32 = 2048. The only subnet mask that gives subnets of this size is 255.255.248.0 . NET 413 HW #2 1

b) (8 pts) Out of these 32 subnets, list the Subnet ID of any 2 subnets and the first and last valid host address in each of these 2 subnets. The subnet ID bits increment by 1 to identify the next subnet. For example, if the subnet ID field uses 3 bits, then the subnet IDs are 000, 001, 010, 011, 100, 101, 110 and 111 (i.e., 0-7 in decimal). Recall that the first address in a subnet is the subnet address (all zeroes in host ID) and the last is the broadcast address (all ones in host ID). i) 1st subnet: Subnet ID = 177.248.0.0/21, First valid host IP = 177.248.0.1, last valid host IP = 177.248.7.254 ii) 2nd subnet: Subnet ID = 177.248.8.0/21, First valid host IP = 177.248.8.1, last valid host IP = 177.248.15.254 iii) 3rd subnet: Subnet ID = 177.248.16.0/21, First valid host IP = 177.248.16.1, last valid host IP = 177.248.23.254 iv) In general, any Subnet ID 177.248.8*k.0/21, for integer k between 0 and 31, is a valid Subnet ID for this problem. 4. Refer to the following diagram, where a switch with ports 1 to 24 is connected to devices with MAC addresses A to H. Assume that the forwarding table is initially empty and that the following sequence of transmissions occurs (each is a one-way frame transmission and there is no timeout of Forwarding Table entries during this time): [1] Device A transmits a frame to device E [2] Device E transmits a frame to device A [3] Device D transmits a frame to device G [4] Device E transmits a frame to device H [5] Device G transmits a frame to device D [6] Device H transmits a frame to device E For each frame transmission, the switch will do two things: a. add the source MAC and incoming port into the Forwarding Table or update the entry if it is already in the table. b. look up the destination MAC in the Forwarding Table. If it is in the table, then the frame is sent out the corresponding port. If it is not in the table, then the frame is broadcast out all active ports except the incoming port. An active port NET 413 HW #2 2

a. (2 points) When STP protocol executes, which switch (S1, S2 or S3) will become the Root Switch for this network? Why? S3 will become the Root Switch because it has the lowest Priority value, which gives it the lowest overall Bridge ID value. b. (6 points) Give the Port Role for each of the following ports, i.e., one of {Root, Designated, Backup}. i. S1 Fa0/1 Designated (lower BID than S2) ii. S1 G0/2 Root iii. S2 Fa0/1 Backup (aka Alternate) (higher BID than S1) iv. S2 G0/2 Root v. S3 G0/1 Designated vi. S3 G0/2 Designated c. (2 points) Which ports in this network will be placed into the Blocking State (also called the Discarding State in RSTP)? S2 Fa0/1 will be placed into the Blocking State because this link is not part of the spanning tree. All other ports, including those for access links, will be in the forwarding state. I did allow for S1 Fa0/1 to also be in Blocking state since it makes sense not to forward across this link. However, the STP protocol says all Designated ports end up in Forwarding state. d. (2 points) What is the Root Path Cost for switch S2? In other words, what is the Path Cost from S2 to the Root Switch? S2 G0/2 is a 1Gbps link so Root Path Cost is 4 . From slide 24 in 4STP: NET 413 HW #2 4

e. Assume that the Host MAC address is 11:22:33:44:55:66. After STP has executed, the Host sends out a broadcast, so that its MAC address is added to the Forwarding Table on each of the 3 switches. i. (2 points) What is the port associated with 11:22:33:44:55:66 in the S2 Forwarding Table? Switch S2 Forwarding Table MAC Address Port 11:22:33:44:55:66 G0/2 ii. (2 points) What is the port associated with 11:22:33:44:55:66 in the S3 Forwarding Table? Switch S3 Forwarding Table MAC Address Port 11:22:33:44:55:66 G0/2 f. (5 points) Suppose you want Switch S2 to become the new Root Switch. What command could you type on the Switch S2 CLI to make this happen? From slide 22 in 4STP: Any of the following would work: spanning-tree vlan 1 root primary (6*4094+1=24577 so less than 28673) spanning-tree vlan 1 root secondary (7*4096+1=28673; however, S2 MAC address is less than S3 MAC address so S2 BID is less than S3 BID) spanning-tree vlan 1 priority <value> where value = (x*4096+1) and is strictly less than or equal to 28673, i.e., x is one of {1,2,3,4,5,6,7}. The values 8-15 would not allow S2 to become Root Switch. NET 413 HW #2 5