class Entity
{
	// ...
	[Column(TypeName = "inet")]
	public (IPAddress, int) Address { get; set; }
	// ...
}

public class CustomNpgsqlTypeMappingSource : NpgsqlTypeMappingSource
{
    public CustomNpgsqlTypeMappingSource(
            TypeMappingSourceDependencies dependencies,
            RelationalTypeMappingSourceDependencies relationalDependencies,
            ISqlGenerationHelper sqlGenerationHelper,
            INpgsqlOptions npgsqlOptions = null)
            : base(dependencies, relationalDependencies, sqlGenerationHelper, npgsqlOptions)
    {
        StoreTypeMappings["inet"] =
            new RelationalTypeMapping[] {
                new NpgsqlInetWithMaskTypeMapping(),
                new NpgsqlInetTypeMapping() };
    }
}

// Basically copied from NpgsqlCidrTypeMapping
public class NpgsqlInetWithMaskTypeMapping : NpgsqlTypeMapping
{
    public NpgsqlInetWithMaskTypeMapping() : base("inet", typeof((IPAddress, int)), NpgsqlDbType.Inet) {}

    protected NpgsqlInetWithMaskTypeMapping(RelationalTypeMappingParameters parameters)
        : base(parameters, NpgsqlDbType.Inet) {}

    protected override RelationalTypeMapping Clone(RelationalTypeMappingParameters parameters)
        => new NpgsqlInetWithMaskTypeMapping(parameters);

    protected override string GenerateNonNullSqlLiteral(object value)
    {
        var cidr = ((IPAddress Address, int Subnet))value;
        return $"INET '{cidr.Address}/{cidr.Subnet}'";
    }

    public override Expression GenerateCodeLiteral(object value)
    {
        var cidr = ((IPAddress Address, int Subnet))value;
        return Expression.New(
            Constructor,
            Expression.Call(ParseMethod, Expression.Constant(cidr.Address.ToString())),
            Expression.Constant(cidr.Subnet));
    }

    static readonly MethodInfo ParseMethod = typeof(IPAddress).GetMethod("Parse", new[] { typeof(string) });

    static readonly ConstructorInfo Constructor =
        typeof((IPAddress, int)).GetConstructor(new[] { typeof(IPAddress), typeof(int) });
}

services.AddDbContext<MyContext>(c => {
                c.UseNpgsql(Configuration.GetConnectionString("Default"));
                c.ReplaceService<IRelationalTypeMappingSource, CustomNpgsqlTypeMappingSource>();
            });