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    DHCP Question...

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    wrcombs dhcp networking
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    • scottalanmillerS
      scottalanmiller @WrCombs
      last edited by

      @WrCombs said in DHCP Question...:

      He is explaining to me that this company Cybera is setting up a firewall for him at his location and is curious why they would leave it that wide and open without any reserved Static IPS.

      Simply not needed. He's imaging that statics do something that they do not. It's common to use statics, and nothing wrong with it. but it is generally considered "less ideal" to do so.

      1 Reply Last reply Reply Quote 1
      • scottalanmillerS
        scottalanmiller @Dashrender
        last edited by

        @Dashrender said in DHCP Question...:

        He avoids static whenever possible. There are times where you should absolutely use static (hypervisors/iLo/iDrac, etc).

        What would make those absolutely static? As long as you have DHCP working, they don't need to be static. When do you want or need any of them during a DHCP outage? DHCP can be fixed in minutes even when HA has failed.

        There is no "absolutely need" statics outside of the gateway and DHCP server.

        1 Reply Last reply Reply Quote 1
        • scottalanmillerS
          scottalanmiller @WrCombs
          last edited by

          @WrCombs said in DHCP Question...:

          .1 isn't reserved, but it's the practice that's most used .
          Understood.

          Reserved "in our minds" 🙂 But not reserved in DHCP terms.

          1 Reply Last reply Reply Quote 0
          • scottalanmillerS
            scottalanmiller @IRJ
            last edited by

            @IRJ said in DHCP Question...:

            Yep I agree with others. Just don't use static IPs at all. If you don't have to use reservations that's even better.

            Even that is getting more and more rare, for sure.

            1 Reply Last reply Reply Quote 0
            • PhlipElderP
              PhlipElder @WrCombs
              last edited by

              @WrCombs said in DHCP Question...:

              This is for a friend of mine who asked me ; And Wanted to be able to send him a link to read up on DHCP Best practices and ideas on his situation.

              He came to me and said "if you set up a dhcp why do you set up .2-.254 with a gate way of .1
              don't you want to keep some open for Static IPs... for example: printers?"

              what can I say to him other than .1 is reserved for gateway? .1 is the gateway so it can't be used in the scenario.

              He is explaining to me that this company Cybera is setting up a firewall for him at his location and is curious why they would leave it that wide and open without any reserved Static IPS.

              I'm sending him the link to this thread to have him read through the answers I get.

              Our rule of thumb, and it's a "we've been doing it this way since ... so we keep doing it this way" situation, is to set up the full subnet in DHCP and then set exclusions for what we want to set aside for servers, printers, and the like. We generally set printers via reservation.

              Here's a simple scope setup in PowerShell:

              Add-DHCPServerInDC
              Add-DHCPServerv4Scope -Name "OUR Local Scope" -StartRange 10.100.10.1 -EndRange 10.100.10.254 -SubnetMask 255.255.255.0
              Add-DhcpServerv4ExclusionRange -ScopeID 10.100.10.0 -StartRange 10.100.10.1 -EndRange 10.100.10.49
              Add-DhcpServerv4ExclusionRange -ScopeID 10.100.10.0 -StartRange 10.100.10.200 -EndRange 10.100.10.254
              Set-DhcpServerv4OptionValue -ComputerName DC.Domain.com -DnsServer 10.100.10.254 -DnsDomain Domain.com -Router 10.100.10.1
              
              
              1 Reply Last reply Reply Quote 4
              • JaredBuschJ
                JaredBusch
                last edited by

                The DHCP range is always the full subnet. That is standard, even if Windows lets you do stupid shit.

                Here is my home router.
                Instead of visibly showing ranges to exclude, outside of windows, you typically tell it hat range to pass out. I'm passing out .31 - .254
                Primary DNS is my PiHole on .4
                Secondary DNS is the router on .1

                ef0668bf-f775-4b9d-af0e-3fa87b13a940-image.png

                Static mapping for things mostly just so I know what they are when looking at pi-hole reports. My Switch on .2, KVM server on .5, and the plex guest on .6, and printer on .7

                11233d73-864f-425c-a11a-8e8856a49f1d-image.png

                KellyK 1 Reply Last reply Reply Quote 0
                • KellyK
                  Kelly @JaredBusch
                  last edited by

                  @JaredBusch said in DHCP Question...:

                  The DHCP range is always the full subnet. That is standard, even if Windows lets you do stupid shit.

                  Here is my home router.
                  Instead of visibly showing ranges to exclude, outside of windows, you typically tell it hat range to pass out. I'm passing out .31 - .254
                  Primary DNS is my PiHole on .4
                  Secondary DNS is the router on .1

                  ef0668bf-f775-4b9d-af0e-3fa87b13a940-image.png

                  Can you clarify something for me @JaredBusch. You stated that DHCP range is always the full subnet, but yours is from .31 to .254. I feel like I'm missing something.

                  JaredBuschJ 1 Reply Last reply Reply Quote 1
                  • JaredBuschJ
                    JaredBusch @Kelly
                    last edited by

                    @Kelly said in DHCP Question...:

                    @JaredBusch said in DHCP Question...:

                    The DHCP range is always the full subnet. That is standard, even if Windows lets you do stupid shit.

                    Here is my home router.
                    Instead of visibly showing ranges to exclude, outside of windows, you typically tell it hat range to pass out. I'm passing out .31 - .254
                    Primary DNS is my PiHole on .4
                    Secondary DNS is the router on .1

                    ef0668bf-f775-4b9d-af0e-3fa87b13a940-image.png

                    Can you clarify something for me @JaredBusch. You stated that DHCP range is always the full subnet, but yours is from .31 to .254. I feel like I'm missing something.

                    DHCP always serves the entire subnet it is defined on.

                    If you tell it the scope is a /24, it serves .1-.254 always.
                    You then subsequently define which part of the scope you want it to hand addresses out on.

                    In windows that is done by "excluding" things.
                    On most other platforms, it is done by telling it what range to supply to clients that ask for an address. Hence the .31 through .254

                    But regardless of what you specify, either as a range to use or range to exclude, DHCP still serves the entire scope.

                    This is why you can make reservations outside of the listed range as in my .7 printer and .10 phone.

                    PhlipElderP 1 Reply Last reply Reply Quote 1
                    • JaredBuschJ
                      JaredBusch
                      last edited by

                      Here is a shot from Windows Server 2012 R2.

                      I have had to explain many times over the years that just because you "exlcude" a range, it does not mean you cannot put a DHCP reservation in that excluded range. Because DHCP is still and always serving the entire subnet defined in the scope.
                      42e53403-2096-44be-8f38-4b11d0fa68cb-image.png

                      DashrenderD 1 Reply Last reply Reply Quote 0
                      • PhlipElderP
                        PhlipElder @JaredBusch
                        last edited by PhlipElder

                        @JaredBusch said in DHCP Question...:

                        @Kelly said in DHCP Question...:

                        @JaredBusch said in DHCP Question...:

                        The DHCP range is always the full subnet. That is standard, even if Windows lets you do stupid shit.

                        Here is my home router.
                        Instead of visibly showing ranges to exclude, outside of windows, you typically tell it hat range to pass out. I'm passing out .31 - .254
                        Primary DNS is my PiHole on .4
                        Secondary DNS is the router on .1

                        ef0668bf-f775-4b9d-af0e-3fa87b13a940-image.png

                        Can you clarify something for me @JaredBusch. You stated that DHCP range is always the full subnet, but yours is from .31 to .254. I feel like I'm missing something.

                        DHCP always serves the entire subnet it is defined on.

                        If you tell it the scope is a /24, it serves .1-.254 always.
                        You then subsequently define which part of the scope you want it to hand addresses out on.

                        In windows that is done by "excluding" things.
                        On most other platforms, it is done by telling it what range to supply to clients that ask for an address. Hence the .31 through .254

                        But regardless of what you specify, either as a range to use or range to exclude, DHCP still serves the entire scope.

                        This is why you can make reservations outside of the listed range as in my .7 printer and .10 phone.

                        When we define the DHCP Scope we can set the delivery IPs define it to 10.100.10.31 - 10.100.10.225 or the like. One does not need to define the scope according to the full subnet whatever that may be.

                        DashrenderD 1 Reply Last reply Reply Quote 3
                        • DashrenderD
                          Dashrender @JaredBusch
                          last edited by

                          @JaredBusch said in DHCP Question...:

                          Here is a shot from Windows Server 2012 R2.

                          I have had to explain many times over the years that just because you "exlcude" a range, it does not mean you cannot put a DHCP reservation in that excluded range. Because DHCP is still and always serving the entire subnet defined in the scope.
                          42e53403-2096-44be-8f38-4b11d0fa68cb-image.png

                          OK - that makes sense since you put it that way. But Windows does allow you to specify things other than the whole subnet, i.e. the whole /24, just like your EdgeRouter does

                          4eb377a5-849b-43c0-9f90-de311c4a2ae2-image.png

                          J 1 Reply Last reply Reply Quote 0
                          • DashrenderD
                            Dashrender @PhlipElder
                            last edited by

                            @PhlipElder said in DHCP Question...:

                            @JaredBusch said in DHCP Question...:

                            @Kelly said in DHCP Question...:

                            @JaredBusch said in DHCP Question...:

                            The DHCP range is always the full subnet. That is standard, even if Windows lets you do stupid shit.

                            Here is my home router.
                            Instead of visibly showing ranges to exclude, outside of windows, you typically tell it hat range to pass out. I'm passing out .31 - .254
                            Primary DNS is my PiHole on .4
                            Secondary DNS is the router on .1

                            ef0668bf-f775-4b9d-af0e-3fa87b13a940-image.png

                            Can you clarify something for me @JaredBusch. You stated that DHCP range is always the full subnet, but yours is from .31 to .254. I feel like I'm missing something.

                            DHCP always serves the entire subnet it is defined on.

                            If you tell it the scope is a /24, it serves .1-.254 always.
                            You then subsequently define which part of the scope you want it to hand addresses out on.

                            In windows that is done by "excluding" things.
                            On most other platforms, it is done by telling it what range to supply to clients that ask for an address. Hence the .31 through .254

                            But regardless of what you specify, either as a range to use or range to exclude, DHCP still serves the entire scope.

                            This is why you can make reservations outside of the listed range as in my .7 printer and .10 phone.

                            When we define the DHCP Scope we can set the delivery IPs define it to 10.100.10.31 - 10.100.10.225 or the like. One does not need to define the scope according to the full subnet whatever that may be.

                            Right, in Windows, you have a starting point and an ending point.
                            I suppose he's talking about the Length indicator here. That Windows will serve that entire /24 as long as it has data in that range to provide, i.e. the scope or reservations outside of the scope.

                            That's cool - I didn't know that, hadn't considered it before.
                            713394cb-0cb5-4bcc-aaa2-88dd5b6c58a7-image.png

                            PhlipElderP 1 Reply Last reply Reply Quote 0
                            • PhlipElderP
                              PhlipElder @Dashrender
                              last edited by

                              @Dashrender said in DHCP Question...:

                              @PhlipElder said in DHCP Question...:

                              @JaredBusch said in DHCP Question...:

                              @Kelly said in DHCP Question...:

                              @JaredBusch said in DHCP Question...:

                              The DHCP range is always the full subnet. That is standard, even if Windows lets you do stupid shit.

                              Here is my home router.
                              Instead of visibly showing ranges to exclude, outside of windows, you typically tell it hat range to pass out. I'm passing out .31 - .254
                              Primary DNS is my PiHole on .4
                              Secondary DNS is the router on .1

                              ef0668bf-f775-4b9d-af0e-3fa87b13a940-image.png

                              Can you clarify something for me @JaredBusch. You stated that DHCP range is always the full subnet, but yours is from .31 to .254. I feel like I'm missing something.

                              DHCP always serves the entire subnet it is defined on.

                              If you tell it the scope is a /24, it serves .1-.254 always.
                              You then subsequently define which part of the scope you want it to hand addresses out on.

                              In windows that is done by "excluding" things.
                              On most other platforms, it is done by telling it what range to supply to clients that ask for an address. Hence the .31 through .254

                              But regardless of what you specify, either as a range to use or range to exclude, DHCP still serves the entire scope.

                              This is why you can make reservations outside of the listed range as in my .7 printer and .10 phone.

                              When we define the DHCP Scope we can set the delivery IPs define it to 10.100.10.31 - 10.100.10.225 or the like. One does not need to define the scope according to the full subnet whatever that may be.

                              Right, in Windows, you have a starting point and an ending point.
                              I suppose he's talking about the Length indicator here. That Windows will serve that entire /24 as long as it has data in that range to provide, i.e. the scope or reservations outside of the scope.

                              That's cool - I didn't know that, hadn't considered it before.
                              713394cb-0cb5-4bcc-aaa2-88dd5b6c58a7-image.png

                              Yeah, that's the ticket. Start IP and End IP settings can be whatever so long as they fall in the subnet mask below.

                              1 Reply Last reply Reply Quote 0
                              • J
                                JasGot @Dashrender
                                last edited by

                                @Dashrender said in DHCP Question...:

                                But Windows does allow you to specify things other than the whole subnet, i.e. the whole /24, just like your EdgeRouter does

                                You can define more than a /24, but not less.
                                In the following pic, you can see the subnet is /24, in the 192.168.100.0 scope.
                                The popout is where you define the DHCP scope to be distributed, not the scope of the subnet.

                                Be sure not to use "Subnet" and "DHCP Scope" interchangeably like M$ does.
                                MS Has you define a scope when you create a new subnet to be served by DHCP, you don't really do anything with the DHCP Scope until you start filling in the Start IP and End IP. No matter how restrictive you make the DHCP Scope (11 to 50 in this case), you will still have a subnet of 255.255.255.0. If you were to make the DHCP scope 192.168.100.1 to 192.168.101.254, you would see the Subnet automatically change to 255.255.254.0

                                66f1e2e3-914b-4cbb-9936-cdbe77b75582-image.png

                                PhlipElderP scottalanmillerS 2 Replies Last reply Reply Quote 1
                                • PhlipElderP
                                  PhlipElder @JasGot
                                  last edited by

                                  @JasGot said in DHCP Question...:

                                  @Dashrender said in DHCP Question...:

                                  But Windows does allow you to specify things other than the whole subnet, i.e. the whole /24, just like your EdgeRouter does

                                  You can define more than a /24, but not less.
                                  In the following pic, you can see the subnet is /24, in the 192.168.100.0 scope.
                                  The popout is where you define the DHCP scope to be distributed, not the scope of the subnet.

                                  Be sure not to use "Subnet" and "DHCP Scope" interchangeably like M$ does.
                                  MS Has you define a scope when you create a new subnet to be served by DHCP, you don't really do anything with the DHCP Scope until you start filling in the Start IP and End IP. No matter how restrictive you make the DHCP Scope (11 to 50 in this case), you will still have a subnet of 255.255.255.0. If you were to make the DHCP scope 192.168.100.1 to 192.168.101.254, you would see the Subnet automatically change to 255.255.254.0

                                  66f1e2e3-914b-4cbb-9936-cdbe77b75582-image.png

                                  News to me?

                                  7ef5d752-f831-4741-8749-3c214c1b0111-image.png

                                  1 Reply Last reply Reply Quote 2
                                  • DashrenderD
                                    Dashrender
                                    last edited by

                                    Exactly - you can definitely declare less than /24
                                    d68e5161-18a8-4f14-a5ac-3dde7c466028-image.png

                                    You can declare anything that's valid for the given CIDR range
                                    My example shows two scopes with /26 subnet

                                    1 Reply Last reply Reply Quote 2
                                    • scottalanmillerS
                                      scottalanmiller @JasGot
                                      last edited by

                                      @JasGot said in DHCP Question...:

                                      In the following pic, you can see the subnet is /24, in the 192.168.100.0 scope.

                                      Where does it show that?

                                      1 Reply Last reply Reply Quote 0
                                      • J
                                        JasGot
                                        last edited by

                                        I guess I am going back subnet school.

                                        scottalanmillerS 1 Reply Last reply Reply Quote 1
                                        • scottalanmillerS
                                          scottalanmiller @JasGot
                                          last edited by

                                          @JasGot said in DHCP Question...:

                                          I guess I am going back subnet school.

                                          I found it, it's in the faded greyed out area under the red rectangle.

                                          1 Reply Last reply Reply Quote 0
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