If you need to have multiple Azure Web Apps with one static IP for outbound calls you need:

1 - Create a Virtual Network
2 - Create NAT Gateway
3 - Create Public IP
5 - Create SubNets for each App
6 - Attach SubNets to VN
7 - Use NAT In SubNets
8 - Enable vnetRouteAllEnabled for each App

We will use bicep for all of it.

Apps

Notice: Apps should be S1 or higher to allow subnets - https://learn.microsoft.com/en-us/azure/app-service/overview-vnet-integration#pricing-details

In you apps you need to enable vnetRouteAllEnabledin siteConfig. Example bicep:

First you need to creat two apps. Example bicep file for app-one:

param location string = resourceGroup().location

resource hostingPlan 'Microsoft.Web/serverfarms@2020-12-01' = {
  name: 'hostingPlan'
  location: location
  kind: 'windows'
  sku: {
    name: 'S1'
    capacity: 1
  }
}

resource appService 'Microsoft.Web/sites@2021-03-01' = {
  name: 'app-one'
  location: location
  identity: {
    type: 'SystemAssigned'
  }
  properties: {
    serverFarmId: hostingPlan.id
    httpsOnly: true
    siteConfig: {
      vnetRouteAllEnabled: true 
    }
  }
}

Network Part

This bicep will create VN, NAT Gateway, Public IP and SubNet for each Web App.

param location string = resourceGroup().location
var appOne = 'app-one'
var appTwo = 'app-two'

resource publicIp 'Microsoft.Network/publicIPAddresses@2021-05-01' = {
  name: 'public-ip-name'
  location: location
  sku: {
    name: 'Standard'
  }
  properties: {
    publicIPAddressVersion: 'IPv4'
    publicIPAllocationMethod: 'Static'
    idleTimeoutInMinutes: 4
  }
}

resource natgateway 'Microsoft.Network/natGateways@2021-05-01' = {
  name: 'natgateway-name'
  location: location
  sku: {
    name: 'Standard'
  }
  properties: {
    idleTimeoutInMinutes: 4
    publicIpAddresses: [
      {
        id: publicIp.id
      }
    ]
  }
}

resource virtualNetwork 'Microsoft.Network/virtualNetworks@2021-05-01' = {
  name: 'virtualNetwork'
  location: location
  properties: {
    addressSpace: {
      addressPrefixes: [
        '192.168.0.0/16'
      ]
    }
    subnets: [
      {
        name: 'subnet-for-${appOne}'
        properties: {
          addressPrefix: '192.168.0.0/24'
          natGateway: {
            id: natgateway.id
          }
          delegations: [
            {
              name: 'delegation'
              properties: {
                serviceName: 'Microsoft.Web/serverfarms'
              }
            }
          ]
        }
      }
      {
        name: 'subnet-for-${appTwo}'
        properties: {
          addressPrefix: '192.168.1.0/24'
          natGateway: {
            id: natgateway.id
          }
          delegations: [
            {
              name: 'delegation'
              properties: {
                serviceName: 'Microsoft.Web/serverfarms'
              }
            }
          ]
        }
      }
    ]
  }
}

resource prodcutsToSubnet 'Microsoft.Web/sites/networkConfig@2022-03-01' = {
  name: '${appOne}/virtualNetwork'
  properties: {
    subnetResourceId: virtualNetwork.properties.subnets[0].id
    swiftSupported: true
  }
}
resource webhooksToSubnet 'Microsoft.Web/sites/networkConfig@2022-03-01' = {
  name: '${appTwo}/virtualNetwork'
  properties: {
    subnetResourceId: virtualNetwork.properties.subnets[1].id
    swiftSupported: true
  }
}

Now all outbound connections from apps will use one static IP address.

Azure Web Apps with One Static IP Address via Bicep

Apps

Network Part